Left to right: An oil fire, likely set for defensive purposes, burns near the Iraqi capital; Marines on a scouting mission in central Iraq; a British tank on the move in the south

A top Kurdish military official, Rostam Kirkuki, said the Americans bombed the entire corridor between Chamchamal and Kirkuk, a key oil center. 

An American officer confirmed Monday that U.S. forces have been in northern Iraq for about 24 hours. He would reveal no details or numbers of the troops. 

U.S. Marine Col. Keith Lawless, speaking to reporters before a news briefing in the city of Salahuddin in Iraq's Kurdish autonomous region, said the American forces had arrived but would not say from where they had come nor where they were. 

Asked if they had arrived by air, he said reporters should "not presume" they were flown in. 

To the south, British troops were engaged in artillery exchanges with Iraqi forces on the outskirts of Basra, British military officials said. British troops have remained outside the city, the second largest in Iraq, unable to move through it because of pockets of resistance. 

A British spokeswoman at U.S. Central Command said the resistance was coming from irregular units, either the elite Republican Guard, Special Security Organization forces or Saddam's Fedayeen, the Baath Party paramilitary organization. 

The troops moving on Karbala made their rapid advance under heavy allied air protection that wiped out a column of charging Iraqi armor and sent some of Saddam Hussein's outer defenses withdrawing toward the capital. But the weather - not Iraqi troops - halted the long columns of thousands of vehicles that were stretched across the desert and farms. 

The trip north passed bombed anti-aircraft guns, empty foxholes and berms dug for tanks that had been abandoned by Iraqi forces. 

Outside the Shiite holy city of Najaf, south of Karbala, U.S. soldiers skirmished with Iraqi forces before dawn Monday. Iraqis shot rockets and anti-aircraft guns at the Americans. 

Small groups using pickup trucks or on foot tried to approach U.S. positions but were driven back by tank and artillery fire. 

In the North, warplanes continued to fly overhead after the first wave of bombings that struck the Bani Maqem barracks, close to the line that separates the Kurdish-held area, including Chamchamal, from territory under the control of Iraqi President Saddam Hussein. 

There have been reports in recent days of aircraft landing in the region at night. 

At least six bombs struck Iraqi positions Monday with such force that the ground shook three miles away in the city of Chamchamal. 

The few residents who had not yet fled started to pack up and leave. Vehicles of all sizes poured onto the main road out of the town. 

"People are evacuating, but not because of the bombing. They are afraid Saddam will respond with chemical weapons," said Ahmad Qafoor, a schoolteacher. 

Over the weekend, U.S. air strikes in northern Iraq pounded positions of the militant Ansar al-Islam group, an Islamic group with alleged al-Qaida and Baghdad ties. 

To the southeast near An Nasiriyah, a convoy of hundreds of vehicles - including tanks, TOW missiles and armored personnel carriers - was backed up along the road leading to a pontoon bridge across the Euphrates River. 

Two bloody battles a day earlier near An Nasiriyah, 230 miles from Baghdad, had deepened the Marines sense of just how treacherous the drive to the Iraqi capital could be. Some of the Americans had been killed by Iraqis pretending to surrender. 

In London, Defense Secretary Geoff Hoon sought to downplay concerns that the U.S. and British forces were becoming bogged down. 

"I think that within three days of real military operations beginning, the idea that somehow people are losing confidence or heart is nonsense," Hoon told a news conference. 

In the southern desert, where some of the fiercest fighting has taken place, Marines stormed squat adobe cinderblock buildings. They found no one there, but discovered abandoned clothing for chemical or biological attacks. 

People who had been in the buildings departed so quickly they left their boots behind as well as a relatively new picture of Saddam Hussein. 

U.S. troops were digging in with long lines of amphibious armored vehicles stretching across the desert, disguised by camouflage. 

At one position Marines constructed a .50 caliber machine gun nest to cover three buildings in the near distance. 

Outside the Shiite holy city of Najaf, at the northern end of the advance, U.S. soldiers skirmished with Iraqi forces before dawn Monday. 

In an apparent indication of renewed Iraqi resistance in the south, the U.S. military canceled a news media trip to Iraq's most productive oil field, which allied forces previously claimed to have secured. Marine Capt. Danny Chung said the Rumeila oil field was "unsafe" Monday. He gave no details, but there have been news reports of Iraqi attacks on the field. 

Also, a trip for reporters to the southern city of Umm Qasr, where there was sporadic fighting days after the allies took effective control, was canceled as well. 

Officials would not say when they expected to arrive at the capital city. "We'll arrive in the vicinity of Baghdad soon, and I prefer to leave it at that," said Lt. Gen. John Abizaid of U.S. Central Command. 

Because of the resistance at An Nasiriyah, Marine officials said they expected to sidestep the city rather than fight to capture it - the same strategy they employed in Basra.