U.S. forces edged closer to Baghdad on several routes Thursday, clashing with Iraqi troops and preparing for a possible confrontation with Saddam Hussein's Republican Guard.

Vicious sandstorms that had made troop movements nearly impossible abated early Thursday -- a change in fortunes for allied ground and air forces hindered for days by high winds and dense dust.

Under sunny skies outside Karbala, 50 miles southwest of the capital, small groups of Iraqi armored personnel carriers probed Army defenses but were hit by U.S. warplanes before getting within 10 miles of American troops.

"I can't believe they keep doing this. It's suicide to come at us like this," said Lt. Eric Hooper of Albany, Ga.

He and the other troops of Alpha Company, 3rd Batallion, 7th Infantry Division awoke at dawn Thursday to bombs shaking the ground. The men cheered as they watched U.S. planes destroy two Iraqi vehicles on the horizon.

Other forces were on the move.

In the lower reaches of the Euphrates, vast columns of U.S. military vehicles -- one 10 miles long -- rolled along a six-lane highway.

On Wednesday, the 1st Marine Expeditionary force, preparing to push north despite the sandstorm, received a warning from a military intelligence officer: units of the Republican Guard -- Saddam Hussein's best-trained, best-equipped and most tenacious fighters -- were moving south.

He said the units were in a 1,000-vehicle convoy on Highway 7, one of the main routes to Baghdad.

The Iraqis, analysts said, likely were taking advantage of the sandstorms to reposition their tanks in response to U.S. forces approaching the outskirts of the capital.

In northern Iraq, just before midnight Wednesday, about 1,000 U.S. Army troops from the 173rd Airborne parachuted into an air base in the Kurdish autonomous zone, the first large ground force in the region from which war planners want to open another front against Saddam's regime.

And in the south, earlier Wednesday, coalition aircraft pounded a convoy of Iraqi tanks and armored vehicles streaming out of the besieged southern city of Basra, British military sources said. The city has been ringed by British troops trying to secure the city and deliver humanitarian aid to trapped residents.

Marines were fighting house to house in Nasiriyah, 90 miles northwest of Basra. A reporter for WTVD in Durham, N.C., attached to the Camp Lejeune Marines, said at least 25 Marines had been injured. He said Marines were using flares to light areas so they could see their enemy.

One military analyst, asked about the southern advance of Iraqi troops believed to be Republican Guard, called it a bold move -- one that could not have been attempted if American tank-killing A-10 Warthogs and Apache attack helicopters had been able to fly.

"It's not good news," said the analyst, retired Army Gen. John Abrams. "It means [the Iraqi] command and control is working, that electronic warfare has not impacted the command and control, and they are able to reposition in a timely way."

U.S. officials gave conflicting reports about Iraqi troop movements. Brig. Gen. Vincent Brooks, in Qatar, said, "We've not seen any significant movements of the type of force" described. He added, though, there were "local positionings and survival positionings" of various units.

Intelligence officials offered the possibility that paramilitary fighters, so-called Fedayeen, had been moving in recent days, traveling in pickup trucks, SUVs and other civilian vehicles.

An Iraqi military communique issued Wednesday reported the first battlefield action by Republican Guard troops, which it said attacked "enemy concentrations" in central Iraq, destroying six armored vehicles and killing "a great many troops."

Another military statement Wednesday night denied that Saddam's special forces were responsible for all Iraqi resistance -- particularly in fighting over the Gulf port of Umm Qasr. It said U.S. officials "were surprised by the ... jihad [holy war], and heroism of the valiant Iraqi army and our great people."

"The enemies, therefore, clashed with the people and army first in Umm Qasr, thinking that they were facing fighters affiliated with Republican Guard and the Saddam's Fedayeen in Umm Qasr and in other parts of Iraq."

In Basra, British military sources estimated the fleeing Iraqi column at about 120 vehicles. Again, it appeared the Iraqis had been using the sandstorm that blanketed the region -- this time to sneak out.

British forces have ringed Basra for several days, exchanging artillery fire with forces loyal to Saddam. The British say they are coming to the defense of inhabitants who rose up in the streets against Saddam on Tuesday. British reporters have described citizens rampaging through the streets; Iraq has denied any civil unrest.

Basra had been largely quiet Wednesday, after British forces "neutralized" militia fighters who had lobbed mortars at residents on Tuesday, said Lt. Col. Ronnie McCourt, a spokesman for British forces in the Gulf.

The unrest came as the British tried to gain control of Basra and relieve the city's trapped civilian population of 1.3 million, which was fast running out of food and was in danger of outbreaks of cholera and diarrhea from contaminated water.