With the Iraqi capital virtually encircled, US army forces spanned out further in the city while marines in thousands of armoured vehicles poured in from the east after clearing road jams at a key bridge crossing.

US tanks battled across Baghdad's main presidential compound -- a symbol of Saddam's 24-year iron-fisted rule -- amid heavy exchanges of tank, artillery and gun fire on day 20 of the war to oust President Saddam Hussein.

Mystery surrounded the fate of the Iraqi leader after US strikes Monday obliterated a building where he was believed to be meeting with his two sons.

Fresh waves of airstrikes pounded the southern and southeastern fringes of the city, while in the centre, two US tanks captured a key bridge over the river Tigris, where they met stiff resistance from Iraqi forces.

US President George W. Bush, meeting his chief ally British Prime Minister Tony Blair in Belfast, said he did not know if Saddam had been killed in the US bomb attack Monday.

"I don't know whether he survived... The only thing I know is that he is losing power... Saddam Hussein will be gone," Bush said after his third meeting with Blair in as many weeks,.

With focus shifting to a post-war Iraq, Bush pledged the United Nations would have a "vital role" to play in rebuilding the shattered country, denying a reported split over the UN's role in supervising an interim Iraqi government.

Britain reportedly wants the UN to oversee any interim Iraqi military administration while Washington seeks initial US-British military control.

US television channel MSNBC quoted US officials as saying they believed Saddam and his two sons may have been killed, but Pentagon officials would not confirm the report.

Three journalists were killed in US assaults in the Iraqi capital.

Ukrainian cameraman Taras Protsyuk from the Reuters news agency and Jose Couso, a cameraman from Spanish televison station Telecinco, were killed when a US tank fired on the Palestine Hotel where foreign journalists are holed up.

The US military said the tank had fired a single round at the hotel after coming under grenade and small arms fire from the building. Journalists in the building strongly denied that there was any gunfire coming from the building.

Three other Reuters staff members were also injured in the attack.

A correspondent for the Al-Jazeera Arabic television network was killed and a cameraman was injured after the station's offices were hit in an separate attack that the Qatar-based network charged was a deliberate US strike.

The US military denied deliberately hitting Al-Jazeera.

Spain said it would ask the United States for an explanation of the Spanish cameraman's death, which brought to at least 12 the number of journalists and staff killed in the 20 days since the war broke out.

As the fighting raged in Baghdad, Bush and Blair ended a two-day summit in Belfast, where the British leader proclaimed Saddam's regime was now coming to an end.

"In all parts of the country our power is strengthening, the regime is weakening and Iraqi people are turning towards us," he said.

A US commander said that an A-10 Thunderbolt strike aircraft was shot down over Baghdad and crashed, but the pilot ejected and was rescued.

Thousands of armoured vehicles were pouring into the capital, with US military officials saying the end of Saddam's clutch on power was nigh.

"We just continue to seize the initiative, will continue to push. Hopefully the regime will fall. It's just a matter of time," said Major Mike Birmingham, from the US infantry.

US forces continued to fan out across Baghdad, and were only a few kilometres (miles) from encircling the city, officers said.

But Iraqi Information Minister Mohammad Said al-Sahhaf remained defiant, telling journalists US forces would surrender "or be burned in their tanks."

Hundreds of families fled the intensive bombing, driving eastwards in cars, trucks and minibuses overflowing with mattresses, kitchen utensils, beds and food, AFP correspondents reported.

"I'm closing the house and leaving with my family for a safer place. I will come back every now and then to see if something happened," said Ali Rishek, 53, before driving away with his wife and their three children.

The chief of staff of British forces in the Gulf warned of a potential "final act of defiance" by Saddam, with the collapse of his regime looking "inevitable."

"There is always the possibility that they were able to organise some final act of defiance -- and we've got to keep on our guard against that," said Major General Peter Wall.

The fate of Saddam continued to tease and taunt the United States. "We just don't know who might have been killed," one Washington official said after Monday's air assault in Baghdad.

"Obviously we hope that some part of the leadership was taken out of action, but we don't know at this point who might have been there at the time the ordnance arrived," said the official, who asked not to be named.

Witnesses reported that at least 14 civilians were killed in the bombing that destroyed four houses and left a crater eight meters (26 feet) deep and 15 meters wide.

In southern Iraq, a spokesman for British forces said "a couple more days" were needed before the second largest Iraqi city of Basra could be declared secure, a day after Britain has said the battle for the city was largely over.

While some Basra residents ripped down murals of Saddam after the British entered, thousands looted public buildings and the homes of Baath party members.

But one Basra resident told AFP: "The people in Basra feel defeated. Sure, we certainly hated Saddam but we also hate the British and Americans."

A US military spokesman dismissed as a "false alarm" earlier fears that US forces had been exposed to mustard gas while in an Iraqi ammunition storage building near the central city of Najaf.

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