U.S. troops thrust to within 10 km (six miles) of the southern edge of Baghdad on Thursday, and were preparing to fight for control of the city's airport, U.S. officials said.

Iraq was reported to be moving elements of four elite Republican Guard divisions southwards to defend the city, setting up what could be a final showdown for the Iraqi capital.

U.S. military sources said that advance armoured units of the 3rd Infantry Division had met less resistance than they had counted on as they raced towards the city's outskirts.

"We're pushing on really fast," said Captain Kevin Jackson of the division's Engineer Brigade. "There doesn't seem to have been much opposition so far."

At U.S. war headquarters in Qatar, officials said elements of four elite Iraqi Republican Guard divisions were seen moving south to defend Baghdad, but had thus far not attempted a full-scale battle with U.S. forces.

"We are engaging them, but we don't yet have any direct confrontation with the Republican Guard divisions as a whole," Captain Frank Thorp told reporters.

He also said troops were closing in on Saddam International Airport, which is about 20 km (12 miles) southwest of the centre of the sprawling city of five million people.

"Coalition forces at this point are outside of the Baghdad airport and are positioning themselves to engage that fight at a time of our choice," Thorp said.

The city and its outskirts were heavily bombed overnight.

U.S. officials say troops cross 'red line' around Baghdad
U.S. military officials said that the troops had crossed the "red line" around the city, which could trigger a chemical attack by the Iraqis.

"There may be a trigger line where the regime deems [a] sufficient threat to use weapons of mass destruction," said U.S. Brigadier General Vincent Brooks at a briefing in Qatar on Wednesday. "It's a conceptual line across which there may be a decision made by regime leaders."

Backed by fearsome air power, U.S. armored forces moved on the Iraqi capital from two directions. U.S. forces also seized a dam over the Euphrates River northwest of Baghdad.

"What we're seeing is a multi-pronged approach. The noose is quickly tightening around the neck of this regime," said U.S. Lieutenant Mark Kitchens, a spokesman at U.S. Central Command in Qatar.

Black Hawk helicopter shot down in Iraq; 7 killed
A U.S. Army Black Hawk helicopter was shot down in southern Iraq on Wednesday, killing seven of the 11 soldiers aboard, Pentagon officials said.

The helicopter was downed by small-arms fire near Karbala, the site of fierce fighting between the Army's 3rd Infantry Division and Iraqi troops, including Republican Guard forces.

The other four soldiers on board the Black Hawk were wounded and rescued, officials said.

U.S. confirms single-seat Hornet plane down in Iraq
The U.S. military confirmed on Thursday that an F/A-18 Hornet single-seat aircraft went down in Iraq overnight.

"While the coalition does not discuss the details of ongoing recovery operations, we are committed to accounting for all coalition personnel," it said in a statement.

The U.S. Central Command forward headquarters in the Gulf state of Qatar said, in its release headed "Hornet single-seat aircraft down in Iraq," that an investigation was continuing and the pilot's name would not yet be released.

It said the plane came down shortly before midnight Iraq time on Wednesday.

U.S. television networks said the fighter-bomber, based on an aircraft carrier, had been shot down over southern Iraq by a surface-to-air missile.