American Marines and infantry battered through Iraqi forces south of Baghdad on Thursday and were near the international airport, 12 miles from the city center, a U.S. military spokesman said.

According to the reports coming from U.S. Central Command in Doha, Qatar, the feared last stand by Republican Guard troops at the southern gates to the city did not materialize.

"And having seen the opposition we've seen so far, we will be able to get to the outskirts of Baghdad within hours," said spokesman Capt. Frank Thorp.

He said the path to Baghdad was getting clearer as more Republican Guards are defeated.

"We are getting closer and closer to Baghdad. When we decide to go into Baghdad, we will be in Baghdad within a matter of hours from when we decide to go," he said.

Marines began the Thursday drive toward Baghdad protected by withering artillery and mortar fire. A massive convoy of Marines moved along the main road leading to the Iraqi capital.

While some Marine units had been roughly following the Tigris River, the Army was advancing along the Euphrates River.

Relentless fire from 155 mm howitzers rained on Iraqi positions near the town of Numaniyah as the Marine advance resumed. Large black plumes of smoke could be seen rising from the town and its prized Tigris River crossing, taken by American troops Wednesday.

The Marines were stopping every vehicle they met along the main road. Drivers and passengers got out of taxis and cars and stood by the road with their hands in the air.

Black combat boots were discarded along the road by Iraqi fighters who have taken off their military footwear and changed into robes, hoping to avoid capture as combatants.

At a Baath Party building flying Iraqi flags, a small group of men sat clustered in a grassy area around a woman dressed in a black chador and waving a white flag of surrender.

Many groups of Iraqis sat down by the roadside, waving and smiling at the Marines to show they were not combatants.

The Pentagon said Iraqi Republican Guard reinforcements had moved out of Baghdad toward the approaching Americans. New groups of Saddam's best trained and equipped fighters were dispatched to replace guard units shattered Wednesday when some U.S. forces had fought to within 20 miles of the city.

As the Marines closed on Baghdad from the southeast, the Army moved in from the southwest, meeting little resistance as they 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.

The Marine column was moving along the Tigris, joined by thousands of Marines coming in from the west. Multiple convoys, including flatbeds, fuel tankers, first aid and supply vehicles, merged outside Numaniyah, creating a traffic jam at the Tigris bridge.

With coalition forces well into the so-called Red Zone radiating from Baghdad, troops had breached the region in which commanders feared Saddam's forces might resort to the use of chemical or biological weapons.

"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.

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.

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 any, although they've found hundreds of chemical protective suits.

U.S. officials said Wednesday that two of the six primary Republican Guard 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. One hundred Iraqis were captured.

A spokesman for the Baghdad infantry division claimed that only 17 men had been killed and 35 injured since fighting began.

British forces reported progress in the fight for Basra, in the deep south of the country where paramilitary fighters have thwarted attempts to pacify the country's second biggest city since the beginning of the war.

Col. Chris Vernon, spokesman for British military, said reports that the British forces were besieging the city were "hugely inaccurate."

"We are leaving the northeastern side over the Tigris entirely open. ... We are only on the south and to the west, and each of those areas, people are coming in and going out, we are making no attempt to stop them. The word 'surrounding' is inaccurate and the word 'siege' is entirely inaccurate," he said.

In the north, Iraqi forces pulled back at least 3 miles from their major position near Kalak at the Kurdish frontier. The Iraqis abandoned hundreds of bunkers and command centers on Wednesday at this key position 25 miles east of Mosul on the main road from Iribil.

The morning after the pullback, thousands of people from Kalak and the surrounding area streamed into the area, hunting for any usable items and hauling them away.

They took away metal sinks, chairs, pieces of wood and anything of even limited value, undeterred by the unexploded mortar shells and fuses lying around, or the possibility of mines.