The Pentagon has been quoted as saying that air-to-air missiles were fired and one Iraqi jet was seen to crash.

The BBC's Caroline Wyatt in Baghdad: No reaction from Iraq

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

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.

Latest incident

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.

Speaking on Tuesday, Iraqi air force chief General Khaldoun Khattab Omar said: "Iraqi fighter planes are totally free to fly the skies of Iraq, in the north and the south.