U.S. ground troops overran Baghdad's outer defenses today in a swift attack along two fronts that drew only patchy resistance from Iraqi soldiers and catapulted the American invasion force to within striking distance of President Saddam Hussein's nucleus of power.

Lead elements of the Army's 3rd Infantry Division approached from the southwest, sweeping around the city of Karbala, crossing the Euphrates River and charging up a four-lane highway with M1 Abrams tanks and M2 Bradley Fighting Vehicles to within 30 miles of the capital. A regiment from the 1st Marine Expeditionary Force surged from the southeast, meanwhile, passing by the city of Kut, traversing the Tigris River and pushing to within 60 miles of Baghdad.

The proximity of the twin advances to the Iraqi capital and the apparent fade of Republican Guard divisions assigned to defend it set the stage for what U.S. military leaders have described as the climactic battle -- and the most dangerous -- in the two-week-old war aimed at destroying Hussein's three-decade-old Baath Party government.

"The dagger is clearly pointed at the heart of the regime right now," said Army Brig. Gen. Vincent K. Brooks, a spokesman for U.S. Central Command regional headquarters in Doha, Qatar.

But the smooth advance did not come without cost. Two U.S. aircraft were reported down tonight, possibly claiming as many as eight American lives.

Senior U.S. commanders expressed confidence that their troops have crushed one of the six Republican Guard divisions ringing Baghdad, severely damaged two others and effectively cut off the capital from the southern and eastern sections of the country. They also reported signs that Hussein's control over his forces appeared to be weakening under relentless bombing.

But the same commanders also voiced fear that, with U.S. troops pulling so close to Baghdad, Iraqi generals might now order a chemical weapons attack against the American front-line units, a concern exacerbated by wind pattern forecasts and unconfirmed intelligence reports. U.S. Marines, already wearing chemical protection suits, were ordered to use protective boots as well and to keep their gloves and masks ready.

The relative ease with which Army and Marine units pushed toward Baghdad surprised U.S. commanders, who had expected a tougher fight from the Republican Guard divisions that are considered the bulwark of Hussein's defense -- better trained and equipped than the regular army units that U.S. forces encountered in the south. Although some small Guard units of 100 or 200 men opened fire with assault rifles and mortars before running away or being killed, American troops rolled through most Iraqi defensive positions unchallenged and found little but abandoned equipment, according to reports from the field and commanding officers.

The heaviest reported U.S. losses resulted from air operations.

An Army UH-60 Black Hawk helicopter was shot down near Karbala at about 7:30 p.m. local time (11:30 a.m. EST), apparently by small-arms fire from Iraqi defenders, a Pentagon official said. The official said early indications from the scene suggested seven of 11 persons aboard were killed and the four others were rescued by an Army search team. But a spokesman for the Central Command in Doha said only six soldiers were aboard and that the cause of the crash was unclear. He would not confirm any casualties, saying search-and-rescue operations were still underway.

A Pentagon official also said that an F/A-18 fighter plane flying a mission from the USS Kitty Hawk went down "over hostile territory" at 11:45 p.m. (3:45 p.m. EST) but would not give a specific location nor say whether it was shot down. The fate of the pilot was not known.

Commanders Cautious

Most of the Baghdad Infantry Division stationed near Kut and parts of the Medina and Nebuchadnezzar divisions north of Karbala were described as "combat ineffective" by U.S. military officials, who credited the withering air campaign and artillery barrages of the past week.

But some ground commanders said that conclusion was premature, warning that it was not certain whether Republican Guard troops pulled back into the two cities the Americans bypassed -- Karbala and Kut -- or whether they had withdrawn into Baghdad's sprawling southern suburbs, where a sea of boxy concrete homes and cavernous government buildings could provide cover for thousands of soldiers.

"There's an old military saying: If your attack is going well, it's an ambush," said Col. Larry Brown, the chief operations officer for Lt. Gen. James T. Conway, the top Marine commander in Iraq.

It was not clear whether front-line Army and Marine units would continue to close in on Baghdad or would wait for reinforcements before beginning a decisive battle with Republican Guard troops dug in nearer to the capital. But several Pentagon officials suggested U.S. forces would continue to forge ahead because of a belief among senior defense officials that today's advance effectively crushed the Republican Guard's fighting capacity.

"The speed of our guys, the strength has just totally overwhelmed them," one Pentagon official said. "We got inside their decision loop. By the time they were making the decisions to fall back, it was too late."

A U.S. intelligence official said Republican Guard troops were given hasty orders to retreat toward Baghdad, but that under a hail of precision-guided bombs and artillery, the Iraqi soldiers "just couldn't move." Now, the official said, the path to Baghdad is "wide open."

"You've got the initiative now," he added. "Why stop?"

But with thousands of American troops within the "red zone" -- a 50-mile radius around Baghdad where U.S. officials fear Iraq is most likely to use chemical weapons -- further advances could occur more cautiously.

A Surprise Move

The Marines began their assault on Kut Tuesday morning with a bit of subterfuge. Regimental Combat Team 1 of the 1st Marine Division raced up Highway 7 from the south first, in an effort to make Baghdad Division commanders think the entire attack was coming from there. While the Iraqis faced south, Regimental Combat Team 5 pulled a surprise move and turned off a road heading toward Baghdad to circle back and approach Kut from the west.

A third force, Regimental Combat Team 7, then streamed up behind Regimental Combat Team 5 and hit the Baghdad Division from the west while most of its guns and attention were turned south. The pincer left the Iraqis little chance to respond, U.S. officers said. Along the way, Regimental Combat Team 7 captured a key airfield at Numaniyah, which the Iraqis had sabotaged by piling four feet of dirt on the runways.

The Baghdad Division was pummeled from the air in the days leading up to the attack to the point that U.S. intelligence estimated it had only 6,000 of its 11,000 troops and less than 25 percent of its artillery left by the time the Marines hit. After today's assault, Marine commanders revised their thinking to conclude that perhaps the bombing took a greater toll than they thought, either by killing Iraqi soldiers or driving them out of their barracks.

"I think they had a lot of desertion," said Maj. Glindon Ashbrook, the ground watch officer at Marine headquarters. "The regular army units that we've run through [in the south] have shown the [Republican Guard] they don't have what it takes to stop us."

Once commanders deem the Baghdad Division disabled, the only obstacle between the Marines and Baghdad is the Republican Guard's Al Nida Division, which is stationed east of the city and is considered more capable than the Baghdad Division. Airstrikes over the last two days have pounded away at Al Nida in preparation for a possible ground assault. One B-52 bomber alone today dropped 26 500-pound bombs on Al Nida's 41st Brigade.

To the west, the 3rd Infantry Division's assault on the Guard's Medina Division destroyed six T-72 tanks, 13 armored personnel carriers and 15 air defense weapons, according to U.S. Central Command.

U.S. military officials say they believe the Guard's Hammurabi Division is moving south to reinforce remnants of the eviscerated Medina Division. But Iraqi units on the southern and western approaches to Baghdad are now so intermingled that Army intelligence is having difficulty sorting them out, and commanders wonder how much control commanders in Baghdad have over them, the officials said.

As the ground forces advanced, U.S. and British warplanes continued their bombing of targets in and around Baghdad, preparing for the battle to come. U.S. defense officials said American and British aircraft flew more than 1,000 sorties today, many of them focused on striking field forces around the capital.

Journalists in the city heard explosions to the south, from the direction of the Rasheed military barracks, which houses some troops from the Republican Guard and the Special Republican Guard, a select force of between 15,000 and 25,000 soldiers trained in urban warfare that is commanded by Hussein's younger son, Qusay. The Special Republican Guard, which is responsible for protecting presidential palaces and other sensitive installations in Baghdad, is regarded as Hussein's last line of defense.

The city's trade fair complex also was hit by a bomb or cruise missile early today, hurling debris into a nearby maternity hospital run by the Red Crescent. News services quoted hospital officials as saying the airstrike killed several people and wounded at least 25, including motorists whose cars were struck as they drove through the area.

U.S. military officials at Prince Sultan Air Base in Saudi Arabia said coalition aircraft dropped 40 Joint Direct Attack Munitions (JDAMs) on Wednesday in an attack on a storage facility in the Al Karkh district of Baghdad that was used by the Iraqi government, Special Security Organization and possibly the Special Republican Guards. In a separate attack Wednesday night, aircraft used JDAMS to bomb an Iraqi command center on a farm in Radwaniyah, southwest of Baghdad, officials said.

Many Deaths in Hilla

In the city of Hilla, about 60 miles south of the capital, the director of a hospital said 33 people were killed and more than 300 wounded in bombing raids on Tuesday. U.S. military officials said Republican Guard positions around the Hilla area were bombed heavily in preparation for the advance of the 3rd Infantry Division.

A spokesman for the International Committee of the Red Cross in Baghdad, Roland Huguenin-Benjamin, said there were "dozens of smashed corpses" in the hospital, which has been "overwhelmed by hundreds of civilian casualties."

The Pentagon has named 50 U.S. service members killed in action or missing in action, with a little more than 100 injured. The official reports, issued after families are notified, often run behind tallies from the field.

Iraq's state-run television, whose transmitters have been bombed but quickly repaired, broadcast three separate statements attributed to Hussein. One, read by Information Minister Mohammed Saeed Sahhaf, declared that "victory is at hand" and urged Iraqis to continue fighting. "We have only utilized a third or less of our army while the criminals have used everything they brought in," the statement said.

Another warned Kurdish leaders in northern Iraq not to cooperate with U.S. forces. A third offered cash rewards to people who help identify spies for U.S. forces.

Late today, the main state-run channel also broadcast footage of Hussein meeting with Vice President Taha Yassin Ramadan, Deputy Prime Minister Tariq Aziz and several cabinet ministers and aides, although there was no way to determine when the video was shot.

Skirmishes between U.S. troops and Iraqi fighters -- regular army soldiers and militiamen -- continued in cities along the 275-mile supply column stretching from the Kuwait border almost to the outskirts of Baghdad. Some of the most intense combat occurred in Najaf, a city about 90 miles south of Baghdad, where the Army's 101st Airborne Division sought to flush out an estimated several hundred militiamen hiding in residential and commercial areas. Central Command reported that the 101st Airborne destroyed 13 air defense and field artillery weapons systems, as well as six paramilitary vehicles, during combat at Najaf.

U.S. military officials said some Iraqi soldiers and paramilitary fighters in the city had fired on U.S. troops from inside a mosque that is one of the world's most important Shiite Muslim shrines. Brooks, the spokesman in Doha, called the Iraqi offensive from inside the gold-domed Mosque of Ali "a detestable example of putting historical sites in danger" and said U.S.-led forces refused to return fire.

'No-Target' Zones

The allied forces have declared Shiite holy sites in Najaf and Karbala "no target" zones, to be fired upon only in self-defense, he said.

But Sahhaf accused the United States and Britain of trying to destroy the mosques in Najaf and Karbala by buzzing them with warplanes so shockwaves would damage the buildings. In London, British Prime Minister Tony Blair told the House of Commons that U.S. and British military officials believe Iraq may be planning to damage the shrines itself and then blame the United States.

British troops continued to exchange artillery and small-arms fire with militiamen and soldiers holed up in the southern port city of Basra. The British forces, which are assembled to the south and west of the city, have opted not to mount a full-scale advance into the urban center. But commando units have been making regular incursions to gather intelligence and target militia leaders.

Early this morning, U.S. warplanes bombed an intelligence compound in Basra, severely damaging 10 buildings in a complex the size of a city block, U.S. military officials said.

In northern Iraq, Brooks said, Special Forces have been conducting airstrikes against the Iraqi 5th Corps and working with local Kurdish fighters to search a captured camp formerly used by the Ansar al-Islam extremist group, he said. In western Iraq, he said, Special Forces seized the Haditha Dam, which U.S. commanders feared could have been used to flood the Euphrates River to slow the invasion.

U.S. officials announced, meanwhile, that the International Committee of the Red Cross has visited Iraqi prisoners of war at a camp near the port of Umm Qasr. The officials said U.S. and British troops have captured more than 4,500 Iraqi prisoners since the war began March 20.

Baker reported from Marine Combat Headquarters in southern Iraq. Correspondent Rick Atkinson and staff writers William Branigin, Jonathan Finer and Mary Beth Sheridan with U.S. forces in Iraq, correspondent Alan Sipress in Doha and staff writers Jonathan Weisman and Alan Cooperman in Washington contributed to this report.

Soldiers with the 3rd Infantry Division take cover during a firefight in a village across the Euphrates River as the Army continued its push toward Baghdad. On the outskirts of Baghdad, which has been the target of two weeks of heavy bombing by U.S. forces, a man inspects his house, which he said was destroyed by a missile.Capt. Andy MaClean of the 3rd Brigade Combat Team, 3rd Infantry Division rests after a night of fighting in Karbala.