U.S. forces made their deepest thrust into the center of Baghdad today, seizing two palaces built by Iraqi leader Saddam Hussein and a wide swath of the city, as the allies stepped up their searches for prohibited chemical weapons.

The U.S. Army said it had tentatively identified nerve and blister chemical agents in drums discovered in a military compound on the Euphrates River. Commanders cautioned that the initial positive identification of the chemicals must be confirmed by more analysis. In Washington, Defense Secretary Donald H. Rumsfeld warned that "almost all first reports" of chemical weapons are wrong.

But if confirmed, the discovery would provide the first tangible evidence of Bush administration allegations that Iraq has secretly hidden caches of chemical weapons proscribed under terms imposed after the Persian Gulf War of 1991. Disarming Iraq of weapons of mass destruction was a key goal in President Bush's decision to go to war against Iraq.

The chemicals were discovered by a 101st Airborne Division patrol at 9 a.m. in 11 25-gallon drums and three 55-gallon barrels. After initial field tests indicated toxic chemical agents, specially equipped detection vehicles were dispatched to the compound, which lies on the river east of Karbala. Early analyses indicated a high probability of the presence of the nerve agents sarin and tabun and a blistering pulmonary agent believed to be phosgene.

As U.S. troops widened their control over Baghdad on the 19th day of the war, Bush arrived in Northern Ireland and met with his coalition partner, British Prime Minister Tony Blair, to discuss a post-Hussein Iraqi government and other issues. The role of the United Nations in a post-Hussein government was expected to be one area of contention at the meeting. Blair supports a deeper role for the United Nations than does Bush, who seeks a transitional governing authority consisting of Iraqi exiles and people living in the country now.

Secretary of State Colin L. Powell declared, "The hostilities phase is coming to a conclusion."

Powell said Washington would send a team to Iraq this week to begin looking at what is needed to set up an interim Iraqi authority. He said the United Nations can provide humanitarian aid and add legitimacy to the interim authority, but he did not describe a role for the United Nations beyond that. Powell made clear that the U.S.-British coalition should play the leading role.

"The coalition, having taken the political risk and having paid the cost in lives, must have a leading role," said Powell, who commanded forces in the 1991 Gulf War as chairman of the Joint Chiefs of Staff.

Earlier in the day at the United Nations, Secretary General Kofi Annan said he expects the world body to play a key role in rebuilding Iraq after the war and said this would bring legitimacy to the effort. He also announced that he was naming Rafeeuddin Ahmed, a Pakistani national and former associate administrator of the U.N. Development Program, as his special adviser on Iraq.

"I do expect the U.N. to play an important role, and the U.N. has had good experience in this area," Annan said.

Annan intends to visit Britain, France, Germany and Russia this week to see if there is any common ground among feuding Security Council members for a U.N. political role in post-war Iraq, U.N. spokesman Fred Eckhard told Reuters Monday.

On the 19th day of the war,. U.S. forces expanded their presence in the capital city of 5 million people, though sustained some casualties along the southern rim of Baghdad.

An Iraqi missile slammed into the U.S. tactical operations center of 3rd Infantry Division's 2nd Brigade on the southern outskirts of Baghdad, killing at least two American soldiers and two journalists, believed to be from Germany and Spain, military officials said.

Fifteen soldiers also were hurt, which is significant because it is likely to impede the headquarters operation of the brigade. Because the brigade represents a significant portion of the combat strength of the division, that could affect U.S. operations on the ground in Baghdad.

It was unclear whether the Iraqis scored a lucky shot or have direction-finding equipment which detected electronic emissions of the brigade's radios and other communications equipment.

In an accident believed to be of "friendly fire," two Marines were killed when their armored troop carrier took a direct hit from an artillery shell while attempting to cross a bridge over a canal on the outskirts of Baghdad, Reuters reported.

More than 80 U.S. troops are believed to have been identified as killed, taken prisoner or are missing in action since the conflict began April 20, although the official tally often lags behind reports from the field as information works its way to the Pentagon and families are notified. Two American journalists have died in the past few days. U.S. military authorities have said thousands of Iraqi soldiers have been killed in the fighting.

U.S. military leaders had feared that Iraqi forces would employ chemical weapons, especially when they entered Baghdad. Brig. Gen. Vincent Brooks, in a briefing at U.S. Central Command headquarters in Doha, Qatar, said today that the threat has not yet passed even though some soldiers were being allowed to remove their bulky, hot chemical protection gear. He said that decision was being made by tactical commanders on the ground who were able to assess the danger in the area in which they were operating.

Several times in recent days U.S. forces have announced the discovery of suspicious materials but testing for proscribed chemical weapons proved negative.

In the latest case found by the 101st, a scientific "mobile exploitation team" (or MET) based at Udairi air field in northern Kuwait was ordered to the suspicious site, but bad weather grounded the team's aircraft until Tuesday morning, Army sources said.

Yesterday several soldiers close to today's site grew nauseous from a substance initially reported to nerve agents; further analysis determined that the suspicious drum contained a weak form of tear gas.

The MET team is expected to arrive in two CH-47 Chinook helicopters. Procedures call for the team to cordon off the area, conduct field tests, and extract samples. "We're waiting for the MET team, the real experts, to confirm or refute this," a senior officer said tonight.

Commanders at the 101st headquarters, south of Karbala, voiced caution about a rush to judgment.

Meanwhile, Persian Gulf commander Tommy R. Franks, in charge of what the Pentagon calls Operation Iraqi Freedom, visited troops in three areas for the first time as American troops were encircling the capital city of 5 million people to control access.

Brooks said encircling the city would help prevent escape by Iraqi government leaders and keep out any Hussein sympathizers.

He said the sight today of U.S. tanks in downtown Baghdad fostered a growing sense of "cautious optimism" but cautioned against premature exuberance, saying U.S. forces know they still have many battles ahead. He said the Iraqi military still has some military power and command and control ability but no overarching structure of control over operations. He said Iraqi fighters have put up occasionally impressive resistance in the face of the U.S. advance in Baghdad.

"Some of the fights have been fights that are worthy of respect for forces that unfortunately may be dying for a regime that does not have a future, and we take that into account. We don't take for granted that there still will be some actions in a variety of places," Brooks.

Explosions rocked Baghdad today amid heavy bombing after troops and tanks from the 3rd Infantry Division reached the center of the city and entered the grounds of the sprawling Republican Palace, Hussein's main office and security compound, as well as the smaller Sijood Palace.

Scenes of destruction were visible at the sites, which have come to embody Hussein's three-decade-long grip on Iraq. Tanks were shown rumbling down the city's vast military parade ground, and soldiers toppled a large statue of Hussein astride a horse.

U.S. forces also took up positions around other government installations, including the Information Ministry, the Rashid Hotel and a downtown army base. At the same time, knots of Iraqi soldiers dotted the route to the ministry, some thrusting rocket-propelled grenades and rifles into the air. The streets were otherwise deserted, even of militiamen from Hussein's ruling Baath Party.

The 2nd Brigade of the 3rd Infantry Division entered the city at 6 a.m. with 70 M1 Abrams tanks and 60 M2 Bradley Fighting Vehicles under the cover of pilotless drones and A-10 Thunderbolt IIs, the tank-busting planes popularly known as "Warthogs." The brigade moved up Highway 8, fighting back moderate resistance from small groups of Hussein's loyalists armed with small arms and rocket-propelled grenades.

As the Army columns moved northeast toward the Republican Palace on the banks of the Tigris, Iraqi soldiers fled along the river, some jumping in the water. Journalists in the capital reported hearing mortar and machine-gun fire directed at the U.S. forces from Iraqi defensive positions on the eastern banks of the Tigris.

As night fell, U.S. forces dug in for the night on the palace grounds, though they left other positions in the city and returned to their operational bases in the southern part of the city.

Brooks said the raid on the Baghdad palaces did not net any government leaders or weapons caches though valuable documents may have been recovered, he said.

After U.S. forces had taken the two palace compounds, Iraqi television showed footage of Hussein with top aides though it was not clear when the meeting occurred. The president, wearing military fatigues, was shown sitting in a room with windows and with wall maps behind him. Also seen present at the meeting were his younger son and heir apparent Qusay, Vice President Taha Yassin Ramadan and Deputy Prime Minister Tareq Aziz as well as other senior military officials. Elder son Uday was not shown in the film.

Iraqi officials say that Hussein is still in control of his government, though there has been speculation among U.S. officials that he may have been incapacitated in attacks on Baghdad.

Brooks said today that there was no major hunt by U.S. forces to find Hussein because the war is "not about one man" and that his personal fate is irrelevant to the larger goals of the war. And Rumsfeld said that while Iraqi President Saddam Hussein's whereabouts may not be known, "we do know he no longer runs much of Iraq."

With U.S. forces showing more presence in the capital, officials said that Iraqi civilians in some areas were becoming more cooperative with the American-led coalition, pointing out where Iraqi tanks and missiles were located. At the same time, Brooks said, Hussein retains control over some parts of Baghdad, for instance the area known as Saddam City in the northeastern corner of the capital. He said Iraqi intelligence officers and special security members are active in the largely Shiite neighborhood to ensure it remains under government control.

"A considerable amount of energy has been committed by the regime at making sure that that population does not turn against the regime," he said.

At what has been renamed Baghdad International Airport, Brooks said Iraqi workers were helping coalition forces turn on utilities and restore other services as troops continued to secure the sprawling premises. On the edges of the airport, U.S. forces early today ended an hours-long battle against Iraqi fighters.

Earlier troops had discovered a hideaway inside a VIP building at the airport believed to have been used by Hussein, complete with a rose garden, a hand-carved mahogany door, gold-plated bathroom fixtures and an office with a false door that leads to the basement, where soldiers found weapons.

Outlining recent military operations, Brooks said the U.S. 1st Marine Expeditionary Force continued to isolate Baghdad from east of the Diyala River. Iraqi forces destroyed one of the bridges over the river and placed artillery on the far bank to prevent a U.S. push, he said.

Meanwhile, Army V Corps remained active west and northwest of the city as well as in the city center. Northwest of the capital, U.S. troops prevented Iraqi reinforcements from reaching the city, defeating an Iraqi force with tanks, armored personnel carriers, other armored vehicles, artillery systems and infantry, he said.

In southern Iraq, British commanders said they were in control of most of Basra, the country's second-largest city where resistance had been fierce. A British convoy of up to 75 vehicles and about 700 troops pressed into Basra's old city today. The convoy consisted of jeeps mounted with heavy guns, which are lighter and better suited to urban combat. But not a shot was fired as men, women and children came out on the street, some to welcome their new occupiers and others to simply stand and stare.

"The last 48 hours have been historic for the people of Basra," Air Marshal Brian Burridge, commander of British forces in the Gulf, said today in Qatar.

"After decades under the heel of Saddam's brutal regime, U.K. forces are in the process of delivering liberation to the people of Basra," he said. "There will be some difficult days ahead but the Baathist regime is finished in Basra."

During the Basra offensive, three British soldiers were killed, he said. They are among 30 British troops killed either in combat or by accident since the war started. He warned that he still expects British soldiers to face resistance in southeastern Basra, especially in the old city.

Earlier, some Basra residents had gone a looting spree, breaking into a branch of the Central Bank of Iraq and carting away chairs, carpets and tables. At the Sheraton Hotel, people loaded up carts, junked vehicles and any other transport they could find with chairs, sofas -- even the grand piano that had been in the hotel lobby, which residents pushed down the street.

There were conflicting reports in Basra about the fate of Ali Hassan Majid, a cousin of Hussein nicknamed "Chemical Ali" after he ordered the use of poison gas on Kurdish villages in rebellious northern areas in 1988. Some reports said that his body was found after an air strike on his house in Basra in southern Iraq, but Brooks said he could not confirm it and later he was reported alive but cornered by British troops.

In northern Iraq, U.S. troops have succeeded in forcing some pro-government paramilitary forces to withdraw from the area of Mosul but Baath Party fighters remain, Brooks said. "We have not conducted significant combat operations into that area, but we continue to focus targeting on forces that are in the Mosul region as well as other places along what has been described at the green line," he said.

During the briefing, Brooks said the U.S. military had discovered missiles during operations inside Iraq, mostly "in the Al Samoud/Ababil family of missiles," with ranges that can strike neighboring countries and all of which are capable of carrying chemical and potentially biological munitions.

Brooks said coalition forces had control of 900 of the 940 oil wells south of Baghdad, and had extinguished one of two that had been burning.

Rumsfeld said that the new presence of Ahmed Chalabi, an expatriate anti-government leader who was flown to southern Iraq aboard a U.S. military transport along with several hundred militiamen did not suggest that he and his opposition group, the Iraqi National Congress, would automatically be given a broader role in post-Hussein Iraq.

"No, I wouldn't think so," he said, " . . . IThe Iraqi people will sort this out when it is over."

Meanwhile, Ahmed Chalabi, an Iraqi opposition leader was flown aboard a U.S. military transport along with several hundred militiamen to a base near Nasiriyah, a key crossroads along the Euphrates River about 100 miles north of the Kuwaiti border, a Pentagon official said. Chalabi, who heads the Iraqi National Congress, one of a number of U.S.-endorsed opposition groups, planned to help U.S. forces recruit support among the population, the official said.

Correspondents Thomas E. Ricks and Alan Sipress contributed to this report.