Exchanges of artillery and automatic weapons fire were taking place, the witnesses said, adding that helicopters were overflying the governor's offices.

A Canadian journalist "embedded" with the US marines, Matthew Fisher, told CNN 250 US armoured vehicles had entered the city and 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.

"We have forces in Tikrit" a spokesman for the US Central Command (Centcom) said at war headquarters in As-Saliyah, Qatar. "We are actively engaging any forces we need to."

Asked to describe the fighting, the spokesman, who asked not to be named, said it was "spotty" but added that "when you are engaged in a firefight it is always fierce."

Fisher said: "It's a very significant attack. They've brought forward a great number of Cobra assault helicopters and Marine F-18s overhead."

He quoted Kelly as saying he has "enough firepower to darken the skies."

Fisher said US forces had been told there appeared to be a core of about 2,500 Iraqis willing to fight in the town, including the elite Republican Guard and Fedayeen militia fighters.

The attack seemed to ignore a call by 22 tribal leaders in Tikrit for an end to US strikes on the city so a peaceful surrender of pro-Saddam militia to US forces could be negotiated, one of them told AFP.

Yussuf Abdul Aziz al Nassari said: "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."

Speaking in company with a number of other tribal chiefs, he added that messages had already been sent to the militia, whose numbers he said he did not know.

A letter drawn up and signed by the tribal leaders read: "We the undersigned heads of clans in Tikrit and its surroundings are agreed not to fight or shed blood, in order to protect our property and allow the American forces, and only them, to enter the town and assume its provisional administration."

It was not clear, however, if this letter had been sent to anyone.

"The people are afraid, they are telling us to go and see the Americans, they want to surrender peacefully," said another dignitary who did not want to be identified.

No militia or Iraqi troops could be seen in central Tikrit earlier Sunday, only a number of armed and extremely agitated residents, who said they wanted to prevent looting that has occurred in every Iraqi city abandoned to the coalition by forces loyal to the Iraqi leader.

The residents, carrying Kalashnikov assault rifles and grenades, told AFP they would surrender to US forces if these were not accompanied by Iraqi opponents of Saddam's regime, notably Shiites and Kurds.

Saying that much of the population of 100,000 had fled, they asked the message to be passed on that Tikrit would not resist the coalition advance.

"Let the Americans come, we will not fight," said Al Ali Musbakh, a retired civil servant, in the main street which had all its shop shutters pulled down.

"But let them come alone, no Kurds, otherwise we will fight to the end."

The AFP reporter had entered Tikrit from the north, along a road that was open and littered with abandoned armored vehicles but not secured by US-led coalition forces.

A bridge over the Tigris river was damaged but crossable, while along the main road of Saddam's hometown ornate palaces used by him and his relatives were devastated.

The fall of Tikrit would all but mark the end of the US-led war to topple Saddam launched on March 20, which has seen every other major centre in the country of 26 million people fall into the hands of the US-led coalition.

Areas around Tikrit and other parts of northern Iraq are now the focus of military operations, and coalition forces still face resistance from "leftovers" of the Iraqi army, the US Central Command said Sunday in Doha, Qatar.

Large parts of the north "have been the focus of our operations in the past days," said Major Rumi Nielson-Green, stressing that Tikrit was only one of several places the US-led forces were targeting.

Earlier, a CNN television news team was fired at after running a checkpoint while leaving the city, which it entered after a local resident, identified as a teacher, said pro-Saddam militias had fled and coalition forces were negotiating a surrender.

An armed guard in the CNN car returned fire in a dramatic exchange broadcast live, and one of the convoy's drivers suffered a light head wound from shattered glass.

The car was fired on a second time with a pistol from a chasing vehicle and a back window was shot out.

Before the US-led invasion of Iraq, Tikrit residents had pledged to give coalition forces a bitter lesson, but by Sunday their mood had changed.

"It's true that people were for Saddam Hussein, but they are not prepared to fight for him," the local dignitary said.

Saddam and most of his top aides have not been seen since before US forces seized Baghdad on Wednesday.

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