"I can confirm they're outside the airport," said Major Randi Steffy, a spokeswoman for the US Central Command in Qatar, said of the facility southwest of the capital.

The aerial blitz against Baghdad showed no signs of abating as dull explosions from the edge of the city continued during the morning after a night of heavy bombardment on its southern and southeastern fringes.

US officers said some 500 Iraqi troops were killed including members of the Special Republican Guard, Saddam's toughest unit, in clashes with US forces for a key bridge some 30 kilometers (20 miles) southwest of Baghdad.

As the war entered its third week the US army's 3rd Infantry Divsion, backed by the 82nd Airborne Divsion, drove towards Baghdad from the southwest. US marines moved up from the southeast.

US commanders reported a breakthrough Wednesday in their campaign to chase Saddam from power, saying they had effectively destroyed the Medina and Baghdad Republican Guard divisions defending the approaches to Baghdad.

"The dagger is clearly pointed at the heart of the regime and will remain pointed at it until the regime is gone," Brigadier General Vincent Brooks told a news conference at US Central Command's forward base in Qatar.

There was no immediate word on overall casualties among the Americans on Thursday, but US officers said two US soldiers were killed in an apparent Iraqi rocket-propelled grenade attack west of the Euphrates.

The US forward headquarters in Doha said earlier a US F/A 18 Hornet fighter and its pilot, based on an aircraft carrier, had gone down Thursday. It did not confirm television reports the aircraft had been shot down.

A US Army Blackhawk helicopter was shot down Wednesday, but there were conflicting reports on the toll, The Pentagon said seven had died, while the US Central Command said only that six were aboard.

Ferocious fighting erupted by the Euphrates River bridge captured by the Americans on Wednesday and targeted by three Iraqi counter-attacks on Thursday, an AFP correspondent said.

US forces hit back with artillery fire from Bradley armoured vehicles and Paladin guns, as well as from A-10 Thunderbolts which flew overhead.

The first counter-attack launched shortly after midnight saw some 200 Iraqi soldiers line up behind a T-55 tank while a second around three hours later saw a similar number of troops take cover behind an armoured personnel carrier.

Another 60 Iraqi troops staged a third counter-attack around dawn but again encountered heavy casualties. Flashes of fire lit up the night sky around the river bank as the Iraqi troops used mortar fire.

"In the attack for this bridge and the counter-attacks, probably 500 died," said Major John Altman, an intelligence officer with the 1st brigade of the 3rd Infantry Division said.

A BBC reporter traveling with the Americans said thousands of US army vehicles crossed the Euphrates heading towards Baghdad after a "fierce fight" to seize a dual-carriageway bridge.

He described a scene of "Iraq prisoners on their knees, with American soldiers standing over them, and Iraq dead in the roadway, and yes, American dead being tended on stretchers."

Having stripped off explosive charges from under the bridge, the US engineers planned Thursday to lay a 150-metre (500-foot) pontoon bridge alongside to allow even more US vehicles to cross the Euphrates, he said.

Southeast of Baghdad, US marines supported by renewed air strikes pushed cautiously towards Baghdad from the town of Kut, around 150 kilometresmiles) away. They braced for a decisive battle.

"We have accomplished every objective quickly and easily. I don't think it's a trap by the Republican Guard -- they're still trying to fight but they have no training, poor equipment and they're very sloppy," said Master Gunnery Sergeant Errol Ovid.

Coalition jets and attack helicopters continued to weed out the Iraqi positions in Kut after a pause during the night when, according to one military source here, they "ran out of targets".

In the southern port of Basra, besieging British forces were still facing resistance from around 1,000 Iraqi militia, along with regular troops who have moved back into the city, a British military spokesman said Thursday.

"There are somewhere around a thousand, but it may be more," Colonel Chris Vernon told reporters when asked the size of the irregular forces within the city.

"It's quite clear that elements of (the Iraqi army's) 51 brigade that we gave an opportunity to capitulate have pulled back inside," he added.

WAR.WIRE