U.S. and British forces moved in on Iraq's second-largest city Saturday, taking its airport and a bridge while Saddam Hussein's security forces resisted with artillery and heavy machine guns.

U.S. forces captured the airport on the north side of Basra after encountering resistance from Iraqi troops in armored personnel carriers, said Marine Lt. Eric Gentrup.

"There was a decent amount of resistance," Gentrup said.

Meanwhile, the U.S. Army's V Corps took Nassiriyah, northwest of Basra, said U.S. Navy Capt. Frank Thorp, a spokesman for Central Command.

At Nassiriyah, the commander and deputy commander of Iraq's 51st Infantry were among those who surrendered Friday night, becoming the highest-ranking Iraqi officials to give up, Thorp said.

The number of those who have surrendered is "in the thousands" and coalition forces have taken about 1,500 POWs, he said.

At Basra, the Americans also took one of several bridges going into the city but British officials said the Iraqis still held other bridges.

While the Marines pressed north, the British took charge of fighting at Basra and said they hoped the city would surrender without a major battle or their having to storm it.

Earlier, jets bombed Iraqi tanks holding bridges, and American and British forces came under artillery fire Saturday as they moved up Highway 80 south of Basra.

Groups of Iraqi soldiers came out to surrender on the highway while others held out against the U.S. and British convoy grinding past blazing oil pipelines and concrete barracks.

Iraqi forces fired artillery in the direction of the U.S. troops but missed their targets. Cobra attack helicopters flew overhead, making their way through clouds of smoke, as coalition forces moved within miles of Basra.

"There's still a little bit of fighting but we're getting there," Thorp said.

British Defense Secretary Geoff Hoon said in London that regular Iraqi forces have withdrawn from Basra but elements of Saddam Hussein's security forces are continuing to resist.

Hoon said Saddam's regime was crumbling under the pressure of a huge air assault. "As last night's dramatic television coverage showed, the lights stayed on in Baghdad, but the instruments of tyranny are collapsing," Hoon said.

Hundreds of tanks and armored vehicles lined Highway 80 -- nicknamed the "Highway of Death" during the 1991 Gulf War when U.S. airstrikes wiped out an Iraqi military convoy fleeing Kuwait.

The roadside was dotted with Iraqi tanks blackened by direct hits on their dug-in dirt bunkers. White flags flew over some deserted, dilapidated barracks, including one where a white cloth had been hung over a picture of Saddam Hussein.

Other barracks still needed to be cleared. U.S. Marines used amphibious assault vehicles to surround clusters of low, crude concrete buildings and shell nearby tanks.

At one of the barracks, Iraqis emerged to surrender, stumbling across a rutted field clutching bags of belongings. As Marines moved toward them, the Iraqis knelt in the field with their arms crossed behind their heads.

Elsewhere groups of Iraqi men in civilian clothes stood near the highway. Allied officers believed they were Iraqi soldiers who had fled their barracks and changed out of their uniforms before the Marines and British forces arrived.

To the rear, other allied troops took custody of throngs of prisoners who surrendered Friday, including members of Iraq's 51st Infantry Division. Captives were being placed in improvised pens of razor wire, watched over by Marines; their partly disassembled rifles were piled beside the road.

The surrendering soldiers were not the elite Republican Guard which anchors Saddam's defense. They appeared to be underfed, ragtag fighters, many of them draftees in T-shirts.

The ground campaign, which began Thursday, appeared to be moving faster than planned. Units reached locations in Iraq 24 hours ahead of their expected arrival time, according to several reporters attached to those units.

The bulk of the allied force hadn't even entered Iraq yet. At the Kuwait border, part of the force was tangled in a massive traffic jam Saturday, with long columns of vehicles waiting to cross the border.

It was unclear whether the allies would try to capture Basra or bypass it now that the region's vital oil facilities appeared to have been secured.

"On the whole, the oil infrastructure appears pretty much intact, beyond the odd bit where they managed to do some damage," said Maj. Charlie Eastwood of the British 7th Armored Brigade.

Elsewhere in Iraq, the U.S. Army's 3rd Infantry Division surged 100 miles through the desert parallel to the Euphrates River, heading straight for the Republican Guard around Baghdad.

The Army's 101st Airborne Division also joined the fight. Its 3rd Brigade was making a marathon trip through wind-swept desert, with soldiers forced to wear goggles and put up with dust in their ready-to-eat meals.

"With all the dust coming in it's hard to breathe," said Spc. Gregory Pagan, 26, of Overbrook, Kan., who had been riding in the back of a Humvee.