Areas of heavy fighting were reported as a tank battalion task force from the US Army's 3rd Infantry Division punched its way along a road from the airport, 20 kilometers (12 miles) to the southwest.

Colonel Will Grimsley said the tanks "looped through the city center" as a show of force a day after US forces seized control of most of the airport, a key prize in their 16-day-old campaign.

"It's called 'let me poke you in the eye because we can and you can't do anything about it'," said Grimsley, commander of the division's First Brigade, adding that the move "started at first light."

"We have had troops that are approaching the heart of the city. They are conducting patrols and operations in Baghdad and have encountered sporadic resistance," said Ensign David Luckett, a spokesman for the US Central Command.

Grimsley said elements of the division's Second Brigade had come under rifle fire and attack by rocket-propelled grenades (RPGs) as they moved towards the heart of Baghdad.

Witnesses told AFP that Iraqi and US forces clashed fiercely for three hours in an area of Baghdad about 10 kilometres (six miles) from the centre.

A series of loud explosions rattled the Iraqi capital Saturday after a night of continuous heavy bombardment, AFP correspondents reported. The blasts were heard from the southwestern edge of the city near the airport.

US forces took control of the airport on Friday -- renaming it Baghdad International Airport -- but were still working to consolidate their hold early Saturday.

Lieutenant Colonel Scott Rutter, commander of the 2-7 Infantry Battalion of the 1st Brigade, said that clashes with Iraqi troops continued around the airport late Friday.

"Between 100-120 enemy" fighters had been killed by his battalion in clashes on Friday afternoon and Friday evening after they launched a counter-offensive, said Rutter.

Six Iraqi T-72 tanks had been destroyed in the exchanges by a combination of javelin laser-guided missiles and other conventional anti-tank weaponry around the sprawling airport complex, he added.

They also destroyed five Iraqi armoured personnel carriers and "teams" of Iraqis firing rocket-propelled grenades.

There was no overall word on American casualties but US officials said two pilots were killed early Saturday when their US AH-1W "Super Cobra" attack helicopter crashed in central Iraq.

They said three other US soldiers were killed in an accident late Friday involving their armoured vehicle at the airport.

Huge fireballs rose over southwestern Baghdad where US forces were securing the city's main airport early Saturday.

Mortar and small-arms duels raged in a corner of the massive airfield hours after US troops and armour smashed through the perimeter fence.

Brigadier General Vincent Brooks, at Central Command's Qatar base, said troops at the airport would be working to clear an underground complex of any Iraqi defenders.

The chief spokesman for Saddam warned of a spectacular counter-attack involving "martyrdom" -- usually a reference to suicide bombing -- to teach the invaders a lesson.

"It will be a great example to them," said Information Minister Mohammad Said al-Sahhaf. "I mean some kind of martyrdom, and there are very, very new ways which we are going to carry it out."

Brooks refused to be drawn on the Pentagon's strategy to take Baghdad, which was at the centre of a steadily constricting noose of coalition ground forces. But he warned the battle would take time.

"We will be very deliberate about how we do our work regarding Baghdad," Brooks said, declining to set a timetable for an assault on the city. "It will take time to gain a degree of control and security over ... all of Baghdad."

SPACE.WIRE