The moves in northern Iraq happened recently, the officials say.

They add there are indications that Iraq may be taking similar measures in southern oilfields.

The American claims were immediately denied in Baghdad.

US officials have previously warned that Iraq may try to set fire to its oil fields, as it did in Kuwait when its forces retreated in 1991.

They have said that, in the event of war, the US administration will seize the oilfields and hold them in trust for the Iraqi people.

Northern front

One of the main concerns for the Pentagon has been how events might unfold in the Kurdish northern regions.

The uncertainty over whether Turkey will allow its territory to be used as a springboard for an attack has so far blocked the US option of creating a northern front.

The Pentagon still wants to put forces in there, but without agreement from Turkey they would have to come from Kuwait or further afield.

The oil cities of Kirkuk and Mosul are held by Saddam Hussein's administration, although the Iraqi Kurds have run their own affairs since 1991.