U.S. forces punched into the heart of Baghdad this morning and took up positions on the western bank of the Tigris River, storming at least two presidential palaces and seizing a wide swath of the capital in a bold daylight assault on the nucleus of Saddam Hussein's government.

Troops and tanks from the U.S. Army's 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. There were unconfirmed reports that U.S. forces also had attacked other government installations, including the Information Ministry, the Rashid Hotel and a downtown army base.

Live television footage from the capital showed dramatic scenes of destruction at sites that 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.

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.

Four or five Marines were killed this morning 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, the Associated Press reported, quoting Lt. Col. B.P. McCoy of the 3rd Battalion, 4th Marines. There were no other immediate reports of American or Iraqi casualties during the incursion.

It was unclear whether the American troops intended to stay in the palaces or to pull back to their staging areas outside the capital. U.S. military officials said the assault was not an effort to occupy the city. A column of U.S. tanks and armored personnel carriers rumbled through the city on Saturday and fought with Iraqi defenders but did not seize any territory.

Col. David Perkins of the 3rd Infantry's 2nd Brigade was quoted by the AP as telling his troops before the operation that the mission was intended to be "a dramatic show of force" to demonstrate that U.S. forces can enter Baghdad at any place and at any time. "I hope this makes it clear to the Iraqi people that this [government] is over and that they can now enjoy their new freedom," Perkins said.

Speaking from the roof of the Palestine Hotel, just across the Tigris from the Republican Palace, Iraq's information minister, Mohammed Saeed Sahhaf, insisted U.S. troops had not entered the city. "There is no presence of the American columns in the city of Baghdad at all," he said. Asked about the nearby presence of U.S. troops, Sahhaf asserted: "We besieged them and we killed most of them. We will slaughter them all."

This morning's assault followed a day in which U.S. forces around Baghdad expanded northward on both flanks of the Iraqi capital and secured major roads leading from the city, tightening their hold on President Saddam Hussein's power base and probing his defenses.

The sweeps provoked several intense engagements but encountered no coordinated resistance. They were undertaken even as U.S. commanders rapidly reinforced their positions around the capital, laying groundwork for what officers had predicted would be a deliberate, step-by-step campaign to strangle Hussein's government holding out in the city center and demoralize the urban guerrilla forces pledged to defend him to the death.

Lengthy convoys of U.S. tanks, armored vehicles and supply trucks streamed into Baghdad's international airport Sunday, turning it into a forward base housing about 7,000 soldiers and growing fast. After night fell, two C-130 Hercules transport aircraft, flying without lights for fear of missile or antiaircraft fire, became the first planes to touch down at the airport -- officially Saddam International, but rebaptized Baghdad International by U.S. troops -- since it was captured by the 3rd Infantry on Friday.

In southern Iraq, British commanders on Sunday ended a two-week standoff in Basra in southern Iraq and sent almost 60 tanks and armored personnel carriers deep into the city, the country's second largest with 1.3 million inhabitants. British forces battled militiamen armed with rifles, mortars and rocket-propelled grenades and eventually seized control of the southern half of the city. By nightfall, the British had set up checkpoints inside Basra for the first time.

The Defense Ministry in London announced that the operation cost the lives of three British soldiers; there were no estimates of Iraqi casualties.

Meanwhile, in northern Iraq, two U.S. jets mistakenly bombed a convoy of Kurdish and U.S. troops, killing at least 21 Kurds and, according to Pentagon officials, apparently killing a U.S. Special Operations soldier. The bombing, near the village of Dibagah halfway between the major oil cities of Mosul and Kirkuk, was the worst known friendly fire incident of the war.

Ahmed Chalabi, an expatriate anti-government leader who has been in the Kurdish-controlled area of northern Iraq, 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.

In another apparent accident, five Russian diplomats were injured while evacuating Baghdad when their convoy was caught in a gun battle between U.S. and Iraqi troops west of the capital, witnesses said. A Russian journalist who was in the convoy said the vehicles were hit by U.S. forces, but the U.S. Central Command said at its regional headquarters in Doha, Qatar, that no U.S. or British forces were operating in the area at the time.

In Karbala, a city about 60 miles southwest of Baghdad that has been another militia stronghold, soldiers from the U.S. Army's 101st Airborne Division used a combination of airstrikes, artillery and small-arms fire to flush out hundreds of paramilitary fighters who have targeted American supply columns heading north. A U.S. brigade commander estimated that between 60 and 100 militiamen were killed and said the rest fled, prompting cheers and waves from thousands of residents. Hundreds of residents subsequently tore down a 25-foot bronze statue of Hussein.

Controlling the Highways

U.S. military officials said the movement of American troops around Baghdad was designed to prevent Hussein's government from reinforcing troops inside the city or fleeing to areas of the country not under the control of U.S. and British forces. The Army and Marines swung around the city in different directions. Elements of the 1st Marine Division began from an intersection in Baghdad's southeastern fringe, about six miles from the city center, that has become their staging area. The Marines traveled to the north and then to the northwest in a counterclockwise arc around the city.

Meanwhile, units from the 3rd Infantry Division moved in a clockwise direction, progressing to the northeast from the division's beachhead at the airport 12 miles west of downtown.

"Look at it from this point of view -- 1st Brigade holds the airport and the west of Baghdad, the 2nd Brigade is securing the south, the 3rd Brigade is holding the northwest and the Marines are in the northeast," Col. William Grimsley of the 3rd Infantry Division told the Reuters news service.

Although American tanks have circumnavigated much of Baghdad, Marine Gen. Peter Pace, vice chairman of the Joint Chiefs of Staff, cautioned that U.S. troops do not control the city's entire perimeter. "To say that you have an impenetrable cordon around the city would be a misstatement," he said Sunday on ABC's "This Week" program.

The sky, he said, is a different matter. U.S. warplanes have started flying over Baghdad around the clock, coordinating precision strikes in advance of ground attacks. "It is certainly true that we have huge amounts of combat power around the city right now, and that we have over 1,000 planes in the air every day," he said. "So if it moves on the ground and it takes aggressive action, it's going to get killed."

The advance of U.S. troops on Baghdad and the movement of tanks around the city were not without resistance. Soldiers with grenade launchers and militiamen in white pickup trucks outfitted with machine guns fired on the American columns, drawing withering return fire from 25mm cannons mounted on M2 Bradley Fighting Vehicles and 120mm shells lobbed by M1 Abrams tanks.

The streets of the capital were largely deserted during the day Sunday and almost totally empty after 6 p.m. -- when a newly instituted curfew took effect -- except for black-clad members of the Saddam's Fedayeen militia, the armed loyalists of the ruling Baath Party.

At his daily news conference in Baghdad, Information Minister Sahhaf argued that Iraqi troops had surrounded the Americans -- not the other way around. "The valiant Republican Guards are encircling the enemy near the airport," he said, asserting that Iraqi forces destroyed six U.S. tanks and killed 50 American soldiers.

As of Sunday, the Pentagon had identified 61 U.S. soldiers and Marines killed in action or missing in action since the war began March 20. Its count lags behind reports from the field, however, as the reporting works its way up the military bureaucracy and families are notified.

'Considerable' Destruction

U.S. military officials estimated that 2,000 to 3,000 Iraqis were killed Saturday during a three-hour incursion by a column of U.S. tanks and armored personnel carriers into central Baghdad. But they acknowledged the estimate is based on the level of resistance and not a body count. Iraqi and foreign journalists in the capital have not seen such large numbers of bodies.

"We know it was a considerable amount of destruction," said Brig. Gen. Vincent K. Brooks, a spokesman for the Central Command in Doha. "In virtually every engagement we have, it's very one-sided."

In an effort to demonstrate that Hussein still is in control of his increasingly chaotic capital, Iraqi state television aired brief footage of the president meeting with his top aides. An announcer said the meeting occurred to-day, but there was no way to independently verify that. The announcer also said Hussein urged soldiers to separate from their regular units to join up with any unit they could find, an indirect acknowledgment of some disarray in Iraqi defenses, and promised that anyone who destroys a U.S. tank would receive a $5,000 reward.

U.S. forces probing around Baghdad continued to find evidence that the Republican Guard has suffered wholesale defections. A Marine light armored unit operating on the east side of the capital on Saturday night ran into 16 T-72 tanks and 29 armored vehicles in the area where the Al Nida Division had been based -- all of them empty. The T-72s are Iraq's most advanced tanks and their abandonment was regarded by U.S. officials as a stark example of the Guard's collapse.

"Where have these guys gone?" said Lt. Col. David Pere, the senior watch officer at Marine headquarters, which has been temporarily moved back to Kuwait in preparation for a jump closer to Baghdad. "The array of forces we thought we'd experience, we just haven't found."

Some Marine units were targeted Sunday by persistent artillery and rocket fire from inside Baghdad. Elsewhere, Iraqi defenders blew up two bridges along the Diyala River on the eastern side of Baghdad in an effort to block U.S. forces, the first time Iraq has destroyed any of its bridges to slow the American advance.

Fighting Non-Iraqi Arabs

U.S. military officials said Sunday that some of the toughest combat that American forces have encountered in recent days has been with non-Iraqi Arabs who have come from other countries to fight. U.S. officers described the foreign Arab fighters as relatively small in number, lightly armed and mixed in with regular and Republican Guard units.

"What we've seen at times were 25 to 30 individuals trying to swarm a light armored vehicle and the light armored vehicle with its weapons was just mowing them down," said Lt. Col. George Smith, a top Marine staff officer. "Some of those that survived the fire would just get back up and continue to charge."

On Saturday night, elements of the 1st Marine Expeditionary Force raided a site that may have been a training camp for non-Iraqi fighters near Salman Pak, a suburban town 15 miles southeast of downtown Baghdad, Brooks said. He said the camp was destroyed in the operation.

U.S. commanders began to breathe easier about the prospect of a chemical weapons attack. Now that their main forces are pressing into Baghdad right up against the remnants of Iraq's troops, U.S. strategists believe the chances of an attack with unconventional weapons have decreased, leading them to downgrade the odds from "likely" to only "possible." As a result, many Army and Marine units were allowed to take off their bulky, sweaty chemical protection suits and required only to keep their gas masks at their sides.

While not finding unconventional weapons, U.S. troops found vast quantities of the regular variety, including four caches with armaments evidently left by deserting Iraqi troops. At one such cache, the Marines said, they found 10 tons of ordnance, including 12 SA-7 surface-to-air missiles. At another, they found 15 surface-to-surface missiles, thousands of rocket-propelled grenades, AK-47 assault rifles, mortars and bombs.

Baker reported from Camp Commando, Kuwait. Correspondents Rick Atkinson with U.S. forces and Alan Sipress in Doha, Qatar, and staff writer Bradley Graham in Washington contributed to this report.

Iraqi soldiers on the banks of the Tigris in Baghdad run away from U.S. tanks that moved on one of Saddam Hussein's presidential palaces.In Karbala, Iraqis pull down a statue of Saddam Hussein with the help of soldiers from the 101st Airborne Division. An Iraqi woman and children are caught in crossfire during fighting in northern Baghdad. U.S. sweeps provoked several intense engagements.An armor task force of the 3rd Infantry Division is on the front line of the Army's advance on northern Baghdad. Defenders who attacked the American columns were answered by fire from M2 Bradley Fighting Vehicles and M1 Abrams tanks.