Gunnery Sgt Frank Leyhew of US Central Command, Florida: "The correct response is to fire."

A US Defence Department official said the plane fired one missile after four British Tornado military jets on patrol near Basra in southern Iraq were illuminated by Iraqi radar.

The Tornados took evasive action and a specially equipped American fighter, an F-16 CJ, was called in.

This carries a high-speed missile known by the acronym Harm (High-Speed Anti-Radiation Missile), which homes in on the radar signal and follows it back to the transmitter.

One Harm missile was fired at an Iraqi anti-aircraft battery.

An official said the Tornados were near Basra in southern Iraq -- within the air exclusion zone which the Allies enforce -- when they detected radar scanning them.

US Defence Secretary William Cohen: "It's too early to tell what their motivation was."

Iraq has flatly denied the American version of events, saying the missile hit a reservoir unconnected to the military.

It said none of its radar had been activated -- and called the US action "aggressive and unjustifiable".

All the aircraft returned safely to base.

Continuing tension

Tornado jets were illuminated by Iraqi radar

A BBC correspondent says the incident illustrates the continuing tension arising from the UN mandated no-fly zones over northern and southern Iraq, in which Iraqi aircraft are forbidden to fly.

There is constant patrolling by Western aircraft, with the constant potential for a clash.

BBC Foreign Affairs correspondent Fergus Nichol explains the history of Iraq's no-fly zone

There are two air exclusion zones over Iraq which were established in the wake of the Gulf War to prevent Iraq from using its air power against its opponents among the Kurdish population in the north and the Shi'ites in the south.

There have been several similar episodes where US aircraft have fired on Iraqi air defences, believing they were threatened, but this is the first incident since 1996, raising questions about its significance.

BBC Correspondent Nick Childs: "Iraq has been recently complaining about the no-fly zone"

The attack comes amid renewed pressure by Iraq for the lifting of international sanctions.