The US troops arrived late on Thursday afternoon and were to be followed later by a second US unit, the official said, speaking on condition of anonymity.

He said Turkish military observers were ready to enter northern Iraq to verify that the American military had taken control of Kirkuk form the Kurds.

"There is no cause for concern," the official said, adding that the Turkish and US authorities were keeping in close contact over developments in Kirkuk.

Turkey has threatened to intervene militarily in northern Iraq if local Kurds are allowed to take over Kirkuk or Mosul, the other main oil city in norhtern Iraq.

On Thursday officials in Ankara warned that any bid by the Kurds to retain permanent control of Kirkuk would be inadmissible.

Ankara fears that if the Iraqi Kurds wrest control of Iraqi oil resources they could seek independence, triggering a similar move among restless fellow Kurds just across the border in southern Turkey.

Washington said on Thursday it would take control of Kirkuk and the Patriotic Union of Kurdistan (PUK), whose fighters had entered the city, said they would leave on Friday and hand over to US troops.

SPACE.WIRE