"Task Force Tripoli has moved north and is currently conducting operations in the vicinity of Tikrit," Captain Frank Thorp said at Centcom's forward command headquarters.

"They have moved north to Tikrit from Samarra, north of Bagdad ... they're on their way, they're moving ... they're in the vicinity," he told journalists.

"It is a significant force with significant firepower," said Thorp, adding that the troops were made up of elements of the First Marine Expeditionary Force.

He would not say whether other US forces were moving toward Tikrit but said the task force was among the troops heading north.

Officials at Centcom declined to say whether coalition forces were negotiating a surrender of the city, the traditional power base of Iraqi President Saddam Hussein.

Fifteen tribal leaders in Tikrit called Sunday for an end to the US bombardment of the city so that the peaceful surrender of pro-Saddam militia there could be negotiated, one of them told AFP.

The area around Tikrit has come under intense coalition bombing over the past week.

On Sunday, there was no evidence of a military presence in the city, but civilians, carried assault rifles and grenades saying they were protecting themselves against looters, and a CNN crew was fired upon at a checkpoint.

Centcom officials insisted Tikrit was only one of several targets of the US and British force.

Large parts of the north "have been the focus of our operations in the past days," said Major Rumi Nielson-Green.

She said the remnants of decimated Republican Guard units had banded together to fight coalition troops.

"Those fights are significant and fierce," she said, adding, however, that the Iraqi units were "just mismatched leftovers with no military command and control".

"A lot of populated areas still have a regime presence," she said of northern Iraq, citing the city of Baiji, 45 kilometers (28 miles) north of Tikrit as one holdout.

"We will work on those until they fall."

Tikrit, 180 kilometres (115 miles) north of Baghdad, is Saddam's birthplace, his tribal power base and a stronghold of his minority Sunni Muslims who have ruled the nation's Shiite majority.

SPACE.WIRE