A US military official said warplanes patrolling the zone responded after being "engaged" by Iraqi jets.

At least one missile is reported to have been fired at the Iraqi planes.

It is not known whether the air-to-air missiles had hit its target.

The spokesman, Major Joe LaMarca, said military experts were still sorting through the reports from the region.

Speaking from Central Command headquarters in Tampa, Florida, he said no allied aircraft were lost. All returned safely to the aircraft carrier USS Carl Vinson.

He described it as the "most aggressive" incident involving Iraqi jets breaching the no-fly zone.

US and British planes patrol the zone, set up after the 1991 Gulf War.

US planes fired on ground missile batteries in that zone and in a northern no-fly zone last week after they said missiles had been fired at them from the ground.

It was the third incident in less than a week involving US warplanes enforcing the no-fly zone.