But US Central Command said its forces would take a cautious approach to the capital itself, as President Saddam Hussein's regime warned of a "non-conventional" attack later Friday on US troops at the airport just outside Baghdad.

Meanwhile three US soldiers were reported killed in a suspected suicide car bombing which also killed two civilians.

Mortar and small-arms duels were continuing in a corner of Saddam International Airport 20 kilometers (12 miles) from Baghdad hours after troops from the US 3rd Infantry Division smashed through the perimeter fence.

US forces said they had seized around 80 percent of the airport and Brigadier General Vincent Brooks, at the command's Qatar base, said it would no longer be called by Saddam's name and was now "a gateway to the future of Iraq."

US troops searched buildings and cleared bunkers while facing ongoing attacks. Brooks said Iraqi troops could be lurking in an underground complex at the airport.

At least dozens of Iraqi troops were believed to have been killed, while US forces said they had sustained no deaths.

An AFP reporter in Baghdad said heavy artillery and other gunfire was heard in the city around 6:50 pm (1450 GMT) for the first time since the war was launched to topple Saddam on March 20.

"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."

He added: "We have penetrated the defensive ring that was set by the Republican Guard forces."

In a televised address to the nation, Saddam urged his people to resist the invasion.

"Hit them with the power of faith wherever they come near you, and resist, oh brave inhabitants of Baghdad," he said. "God willing, you will be victorious and they will be defeated."

Iraqi Information Minister Mohammad Said al-Sahhaf warned there would be a "non-conventional" assault on US troops who were said were "isolated" at the airport.

"It will be a great example to them," he said. "I mean some kind of martyrdom, and there are very very new ways which we are going to carry it out."

Iraq has said it has thousands of volunteers in Baghdad ready for suicide attacks.

US forces said around 2,500 Iraqi troops were taken prisoner after giving themselves up to marines pushing north from the Al-Kut area, but Brooks said he could not confirm the exact figure.

"We have encountered forces that have surrendered along the way. They're usually parts of units, not whole units," he said.

Brooks also said US special forces discovered what they believed was a "training school" for nuclear, chemical or biological warfare.

Bottles labelled with chemical agents were discovered at the building in western Iraq, but initial investigations did not suggest it was a site for the manufacture of weapons of mass destruction.

Overnight coalition bombing of Baghdad plunged the capital into a seemingly intentional power blackout that cut water supplies for the first time in the war. Supplies were restored to parts of the capital Friday night.

In central Iraq, three US troops, a pregnant woman and her driver were reported killed in a car explosion at a US checkpoint.

The circumstances were similar to a March 29 car bomb blast which Iraq said would be followed by others.

A US statement did not call Thursday night's explosion a suicide bombing, but Brooks said: "These are not military actions, these are terrorist actions."

Coalition forces were controlling a checkpoint 18 kilometers (10 miles) from Hadithah Dam in Iraq when a vehicle drove up and a pregnant woman stepped out "screaming in fear," Central Command said.

As soldiers approached, the vehicle exploded. The woman and the car's driver were killed along with three coalition troops. Two other soldiers were injured.

In the south, Brooks said, patrols outside Basra -- where British forces are around the city -- patrols found 56 surface-to-surface short-range ballistic missiles and four missile launchers.

He said British forces were still trying to eradicate "regime death squads" which the coalition says are shooting Iraqi citizens who try to flee or refuse to fight.

In the north, Kurdish fighters seized a bridge near the northern junction of Khazer after more than 24 hours of fierce fighting with Iraqi troops.

Khazer and its Manquba bridge are strategic points in the advance on Mosul, a majority-Arab city of 1.5 million in a mostly Kurdish area some 400 kilometers (250 miles) north of Baghdad.

Brooks said air attacks were pounding Iraqi sites in northern Iraq and that the coalition has "effective control of roads leading into or out of Iraq, and roads between Baghdad and Tikrit", Saddam's home town north of the capital.

He added that coalition strikes with "air assets, sea-based precision-guided munitions and land based long-range fires" combined with the land attacks were "proving to be devastating to Iraqi military forces."

But in his military briefing, Sahhaf said Iraq's elite Republican Guards units had clashed with US forces airlifted to Abu Gharib north of Baghdad, destroying six tanks and three armoured personnel carriers (APCs).

In another clash at Falluja to the west of Baghdad, "our forces launched an attack on one column of the US and British enemy. They stopped and started to flee," said the information minister.

SPACE.WIRE