The Iraqi military came close to hitting a high-altitude U.S. U-2 spy plane with a missile this week, a senior U.S. defense official said yesterday. The official said the modified Russian-made anti-aircraft missile just missed the plane in a "no-fly zone" over southern Iraq on Tuesday.

"It was close to the aircraft," the official said, confirming a CBS News report that the Iraqis had improved their ability to strike at the unarmed surveillance aircraft, which take photographs and obtain other intelligence from an altitude of more than 70,000 feet.

U.S. Defence Secretary Donald Rumsfeld has recently warned that U.S. and British pilots patrolling no-fly zones over northern and southern Iraq faced increasing danger from Iraq's defences.

"We have known that they have had this capability for a long time and we were waiting for when they would try it," the defence official, who asked not to be identified, told Reuters.

The official said the U-2 was almost hit despite the fact that the U.S. military believed the missile was "unguided" - fired without the benefit of targeting radars on the ground.

CBS News reported that the U-2 pilot suddenly saw the missile streaking toward him before it exploded behind and below him. The explosion was close enough for the pilot to feel the shock waves but no damage was caused to the aircraft, the report said.

"There is some level of concern here, but as we have said before, we reserve the right to strike back at a time and place of our choosing," the defense official told Reuters.

U.S. and British warplanes have been patrolling the zones over Iraq for nearly a decade since the 1991 Gulf War. Iraq was banned from using all aircraft, including helicopters, in the air exclusion zones.

No allied aircraft have been lost, although the Iraqi military has repeatedly fired anti-aircraft guns and missiles at the warplanes, which have responded by dropping bombs and firing missiles at Iraqi air defence sites.

The Bush administration is still formulating a policy on Iraq and has been reviewing the no-fly zones and working on revising sanctions against Baghdad.