The seizure of the largely deserted city by US armour marked the last major engagement of the 26-day-old war and came as Washington shifted its attention to seeking out the remnants of the regime and its alleged weapons of mass destruction.

President George W. Bush again hit out at neighbouring Syria, demanding that it "cooperate" with the US-led coalition and stop providing a refuge for fugitive Iraqi officials.

The US armour which poured into Tikrit overnight met with little resistance in Saddam's tribal stronghold 180 kilometres (115 miles) north of Baghdad.

"Around 20 tanks" entered the city centre along the main road from the town of Kirkuk, further north, across a battered bridge over the River Tigris, witnesses told AFP.

Four US helicopters flew low over the main square, an AFP correspondent saw from just outside the centre.

US troops aboard the helicopters landed near the Tikrit headquarters of Saddam's Fedayeen militia, where they also encountered no resistance, the witnesses said.

The only Iraqis visible on the streets of the city of some 100,000 people were a handful of armed civilians intent on protecting their property from looters.

Tribal chiefs had sought to negotiate a peaceful entry by coalition troops, but intermittent air strikes could be heard on the city's outskirts throughout the night.

A Canadian journalist with the US marines, Matthew Fisher, told CNN that the 250 US armoured vehicles which entered the city had met with some resistance from loyalist forces.

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

However the fighting fell well short of the feared last stand by Saddam's closest tribal allies.

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

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."

As US troops advanced on Tikrit, they freed seven comrades who had been captured by Iraqi forces earlier in the war.

Two of the seven were reported to have gunshot wounds but all were "basically in good shape", said Defence Secretary Donald Rumsfeld.

US losses in the war rose to 117 dead Sunday, according to a Pentagon toll.

As US troops completed their occupation of Iraq's main towns, the administration ratcheted up the pressure on Syria, fuelling suspicions that Iraq's western neighbour will be the next target of its attentions.

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.

Rumsfeld said many Syrian nationals were killed in fighting overnight in Baghdad and others had been taken prisoner.

Syrian officials emphatically denied that they were harbouring 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 Mustafa, the number two in the Syrian embassy in Washington.

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 air strike on his farm, west of Baghdad, a family friend told AFP.

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

Top Iraqi nuclear scientist, Jaffar al-Jaffar, also surrendered in recent days, US officials said. His surrender follows that of Lieutenant General Amir Saadi, Saddam's chief scientific adviser, who turned himself in Saturday.

In a warning that the war may not be entirely over, US commanders said marines in Baghdad had uncovered 310 vests designed for use by suicide bombers, with about half of them already 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.

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

Mounting anger at the failure of coalition troops to do more to rein in the looting and lawnlessness that has gripped the capital since the entry of US troops last Wednesday sparked a first anti-US demonstration by some 100 protestors Sunday.

In the main northern cities of Mosul and Kirkuk, an uneasy calm returned after two days of looting and ethnic fighting, as US troops took control from Kurdish rebels in a bid to reassure both the two cities' Arab communities and neighouring Turkey.

SPACE.WIRE