"Kurdish forces entered Kirkuk earlier today with United States special forces with them," Major General Stanley McChrystal, vice director of operations of the Joint Staff, said.

The advance came about after regular Iraqi army forces pulled back, McChrystal said.

"Elements of the 173rd Airborne Brigade in fact followed them later in the morning and are now in the city as well," he said, adding that the US presence amounted to a battalion-sized force.

Asked if US forces had taken control of the strategic, oil-rich city, Mc Chrystal said: "The situation is fluid and has been all day".

"We have United States forces in Kirkuk we talking very closely with the Turkish forces as well as we have been throughout this as they are coalition partner," he added.

Turkey has repeatedly threatened to intervene militarily in northern Iraq if Kurdish forces seized Kirkuk or Mosul, which control some of Iraq's largest oilfields.

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