The US military said it did not know if the air raid on Monday evening had killed the Iraqi president, but said his grasp on the country of 26 million was fast loosening.

The Iraqi capital was virtually encircled and US Army forces spanned out further in the city while Marines in thousands of armored vehicles poured in from the east after clearing road jams at a key bridge crossing. US tanks battled across the city’s main presidential compound — a symbol of Saddam’s 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 center, 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 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 postwar 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. A US commander said that an A-10 Thunderbolt strike aircraft was shot down over this city and crashed, but the pilot ejected and was rescued.

Thousands of armored vehicles were pouring into the capital, with US military officials saying the end of Saddam’s clutch on power was near.

“We just continue to seize the initiative, will continue to push. Hopefully the regime will fall. It’s just a matter of time,” said Maj. Mike Birmingham, from the US infantry.

US forces continued to fan out across the capital, and were only a few kilometers from encircling the city, officers said.

But Iraqi Information Minister Mohammed Saeed Al-Sahaf remained defiant, telling journalists US forces would surrender “or be burned in their tanks.”

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

“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.”