A long convoy of Marines advanced north at dawn Thursday, headed for an expected confrontation with Saddam Hussein's toughest fighters defending the Iraqi capital. A Marine went from Humvee to Humvee asking if troops had enough ammunition.

Iraqi Republican Guard reinforcements were moving out of Baghdad toward the approaching Americans, Pentagon officials said -- apparently replacing guard units shattered a day earlier when U.S. forces fought to within 20 miles of the city.

On Wednesday, Army troops closed on the city from the southwest while Marines did the same from the southeast, meeting little resistance. Marines passed abandoned Iraqi trenches littered with everything from mortars and small arms to teapots and bedspreads.

"When they ran, it wasn't for lack of ammo. They've got enough," one Marine said as he examined the trenches.

Artillery fired at Iraqi positions to the north and east Thursday morning, and there was repeated small arms fire. Howitzers shelled the city of Numaniya, 40 miles southeast of Baghdad, where Marines took a bridge over the Tigris River a day earlier.

Iraqi forces shot down two U.S. aircraft Wednesday. A surface-to-air missile downed a Navy F/A-18 Hornet -- the first American plane shot down in the war on Iraq. There was no immediate word on the fate of the pilot. Near the city of Karbala, 50 miles south of Baghdad, small arms fire brought down a Black Hawk helicopter, killing seven Americans on board and wounding four others.

The Marine column was moving north along the Tigris, joined by thousands of Marines coming in from the west. Multiple convoys, including flatbeds, fuel tankers, first aid vehicles and supply vehicles, merged outside Numaniya, jamming up traffic at the bridge across the river.

Meanwhile, the Army was advancing along the Euphrates River, the closest units reaching within 20 miles of Baghdad -- "able to see the skyline," a senior military official in Washington said.

But confidence over the advance was tempered by U.S. officials' warnings that a cornered Saddam might resort to unleashing chemical or biological weapons.

Lead U.S. infantry units donned their chemical suits after capturing a bridge 40 miles southwest of Baghdad. Some Marines began adding their protective boots to the suits they already wear, and Marine helicopter pilots were advised for the first time to be ready to don chemical suits at a moment's notice -- now that the so-called red zone, the range of guns and missiles defending Baghdad, has been breached.

An officer with U.S. Central Command explained that the "red zone" begins on an imaginary line running east from Karbala, about 50 miles south of Baghdad on the Euphrates River, to Kut on the Tigris River southeast of Baghdad.

"There may be a trigger line where the regime deems sufficient threat to use weapons of mass destruction," said U.S. Brig. Gen. Vincent Brooks.

The United States believes Iraq has mortar shells, artillery and short-range missiles capable of carrying chemical weapons, including the FROG-7 -- used to carry mustard gas during the Iran-Iraq war -- which has a 40-mile range.

Iraq denies it still has weapons of mass destruction, and U.S. troops have yet to locate such weapons, although they've found hundreds of chemical protective suits.

One thing was certain to be waiting for coalition forces in Baghdad: whatever remains of the Republican Guard, Saddam's best-trained and best-equipped forces. U.S. officials said Wednesday that two of the six primary units -- the Medina armored division and the Baghdad infantry division -- had been largely eliminated as an effective fighting force.

Army troops backed by artillery and airstrikes destroyed nearly 100 military vehicles and weapons pieces belonging to the Medina division, including six tanks and 15 air defense weapons, Central Command said in a statement. One hundred Iraqis were captured, it said.

Four other guard divisions are still somewhat intact, with two estimated at 70 percent effectiveness and two somewhat less, according to a defense official, who spoke on condition of anonymity.

A spokesman for the Baghdad infantry division claimed that only 17 men had been killed and 35 injured since fighting began. He said the division was in excellent fighting shape and would teach the enemy "lessons in the art of fighting."

Marines taking the bridge at Numaniya Wednesday met little resistance and found the side of the road littered with abandoned Iraqi military uniforms.

"It looks like a lot of guys threw off their boots and threw off their uniforms and got the hell out," said Lt. Michael Belcher of the 5th Marine Regiment. One man tore off his Iraqi army uniform and donned a brown robe. Marines quickly seized him.

Beside a small marsh, Marines with bayonets fixed on their M-16 rifles stood over a group of 40 Iraqi men sitting on the ground. Four of the Iraqis had their hands bound with white cord. Smoke rose from a building nearby, and the bodies of four other Iraqis lay along the road.

Lt. Belcher said the Marines uncovered "weapons caches throughout the city." And Marines also found Iraqi gas masks still sealed in plastic and displays showing what to do in the event of a chemical or biological attack.

Elsewhere between the cities of Diwaniyah and Kut, thousands of Marines headed out on a route so recently secured that a vehicle apparently hit from the air still had its engine running. The bodies of four Iraqi soldiers were sprawled around it.

Heading north into greener terrain, Marines entering towns were often greeted by Iraqi civilians smiling, waving and selling Iraqi-brand cigarettes.

Southeast of Baghdad, Marines seized the strategic town of Kut and routed the Baghdad division of the Republican Guard, guarding the highway to the capital.

To the west, lead elements of the Army's 3rd Infantry Division pushed through a gap west of Karbala after a night-long bombardment of the Shiite holy city.

The division's First Brigade captured a bridge over the Euphrates River at Mussayib, about 40 miles southwest of Baghdad. The bridge had been rigged with explosives, but engineers defused them.

However, the division's advance slowed to a crawl Wednesday night because of inaccurate maps, creating a miles-long traffic jam of thousands of troops and vehicles lined up to cross the Euphrates.