The planes landed on Tuesday at the Harir airfield near the town of Shaqlawa in the western part of an autonomous Kurdish zone, an area not controlled by Saddam Hussein since 1991.
Air Force General Richard Myers, the chairman of the US joint chiefs of staff, said: "There are not significant forces in northern Iraq right now. I'm not going to go into any more detail on that."
The sightings follow other reports of intensive US reconnaisance and military activity. A US intelligence team visited another airstrip at Bakrajo, in the eastern part of the Kurdish zone last week, where Kurdish labourers and soldiers were preparing it for aircraft, and building infantry bunkers in the area, the New York Times reported.
Bakrajo is a 30-minute drive from the Iraqi frontline, at the edge of a huge military complex which was abandoned when Iraq withdrew its forces in 1991.
Kurdish rebels said they were preparing the Bakrajo strip for war but not necessarily for the Americans. "We are getting prepared for the coming war, getting trained and ready to face any critical situations," said Simko Diyazee, chief of staff for military forces of the Patriotic Union of Kurdistan. "But it doesn't mean America is planning to use this airfield. We are just doing it for ourselves."