US President George W. Bush issued a new demand to Syria that it "must cooperate" with Washington in its efforts to topple Saddam, while again accusing Damascus of holding chemical weapons and harboring runaway Iraqi officials.

Bombing raids by US planes resumed late Sunday around Tikrit, while witnesses said fierce fighting erupted on the outskirts of the city between die-hard Iraqi forces and US troops supported by helicopters.

A Canadian journalist with the US marines, Matthew Fisher, told CNN that 250 US armoured vehicles had entered Tikrit.

He quoted US commander Brigadier John Kelly as saying five Iraqi tanks had been destroyed on the outskirts and at least 15 people killed in firefights.

A spokesman for US Central Central Command in As-Saliyah, Qatar, said: "We have forces in Tikrit. We are actively engaging any forces we need to."

The capture of Saddam's home town, which lies about 180 kilometresmiles) north of Baghdad, would mark a major step towards the end of the three-and-a-half-week-old conflict.

The attack was launched despite an offer from 22 tribal leaders in the city to negotiate a peaceful surrender of the Fedayeen.

Tribal chief Yussuf Abdul Aziz al Nassari told an AFP correspondent in the city: "We are ready to surrender, but let them stop their bombardments. After that we are asking for just two days to persuade the Fedayeen to lay down their arms."

And as Iraqi anger grew over continued lawlessness in their occupied country, local residents staged the first anti-American demonstration in Baghdad since US troops arrived to a warm welcome last week.

The protest by about 100 Iraqis came as US forces began the herculean task of restoring the battered country to normalcy, launching a recruitment drive to put Iraqis back to work in key sectors.

As US forces entered what is expected to be the final military phase of the war, the United States ratcheted up the pressure on Syria, fuelling suspicions Washington will turn its attention next to Iraq's neighbour to the west.

Bush stopped short of threatening US military action against Damascus, but said it must cooperate in efforts to wipe out the remnants of Saddam's regime.

"The Syrian government needs to cooperate with the United States and our coalition partners. It must not harbour any Baathists, any military officials who need to be held to account," Bush told reporters at the White House.

US Defense Secretary Donald Rumsfeld said many Syrian nationals were killed in fighting overnight in Baghdad and others have been taken prisoner.

Syrian officials emphatically denied that it was harboring members of the regime or had weapons of mass destruction.

"We will not only accept the most rigid (weapons) inspection regime, we will welcome it heartily," said Imad Moustapha, the number two in the Syrian embassy in the United States.

General Tommy Franks, commander of US forces in Iraq, reported that some Iraqi leaders had been captured trying to flee the country.

"I will say coalition forces and some Iraqis with whom we have contact have taken some people trying to escape from Iraq," Franks told Fox News.

He gave no details on their identity, but said they were being held in western Iraq.

The Pentagon announced that coalition forces had captured Saddam's half-brother Watban Ibrahim Hasan near the border with Syria trying to flee the country.

Another half-brother, Barzan al-Tikriti, was killed Friday in a US airstrike on his farm, west of Baghdad, a family friend told AFP.

Both Watban and Barzan are on the list of the 55 most wanted Iraqi leaders issued by the US Defense Department.

Meanwhile, seven US soldiers thought to have been captured by Iraqi forces were found near Tikrit, Rumsfeld said. Two of the seven were reported to have gunshot wounds but all were "basically in good shape," he added.

The toll of US military dead in Iraq rose to 117 Sunday, while the tally of US prisoners of war went to zero with the rescue of seven captured soldiers, according to the Pentagon.

US forces began the huge task of restoring normalcy to Baghdad, assisted by hundreds of residents who volunteered to collect the dead, work in policing roles and restore the electricity supply.

The capital, a city of five million people, has been without electricity for about 10 days, and most homes are without running water and telephone services.

A top Iraqi nuclear scientist has surrendered in Iraq, a US official said as US forces stepped up a search for Iraqis who can shed light on the country's weapons of mass destruction programs.

Jaffar al Jaffar, a top nuclear scientist, turned himself in over the past few days, the official said, speaking on condition of anonymity.

His surrender follows that of Lieutenant General Amir Saadi, Saddam's chief scientific adviser, who turned himself Saturday in with the help of Germany's ZDF television network.

In an ominous sign of possible attacks to come, US military officials said Marines in Baghdad had uncovered 310 vests fitted for use by suicide bombers, with about half of them fitted with explosives.

Marines also reported finding five canisters with a substance testing positive for chemical agents but backed off an earlier claim of finding 278 suspect artillery shells.

In the main northern cities of Mosul and Kirkuk, an uneasy calm returned after two days of looting and intercommunal fighting, with US troops reassuring Turkey by replacing Kurdish fighters who captured the oil-rich cities.

British Foreign Secretary Jack Straw and his deputy meanwhile headed for no fewer than six Middle East and Gulf states as the diplomatic race to shape the region's future post-Saddam Hussein heats up.

And US President George W. Bush has postponed his scheduled May 5 visit to Ottawa due to the war in Iraq, the office of the Canadian Prime Minister said Sunday, but dismissed comments it may signal a rift between the two administrations.

Australian Prime Minister John Howard defended the war in Iraq Monday, arguing it was the only way to remove Saddam Hussein and its conclusion will make the world safer from terrorism.

"The whole idea that you could bring about regime change, the whole idea that you could provide the hope and the opportunity for the people of Iraq that has been provided without taking the action that was taken, is ludicrous," he said.

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