These are the first Allied deaths in the seven-week air offensive against Serb forces.

The AH-64 helicopter was 70 kms (40 miles) north-east of the capital Tirana on Wednesday when it came down at 1.30am local time. The Pentagon said there were no reports to suggest it had come under attack.

Task Force Hawk spokesman, Lieutenant-Colonel Garrie Dornan, said they were investigating reports that some of the ammunition on board the helicopter had exploded, though he would not say whether it was before or after the crash.

Lieutenant Colonel Garrie Dornan: "We can never eliminate risk from this business"

There have been reports that the aircraft might have hit power lines.

But the crash will have no effect on the timetable for deployment of the AH-64 against Serb forces in Kosovo, the spokesman said.

Clive Myrie: "Apaches have already been on reconnaisance missions"

This is the second such crash in nine days. Last week, another helicopter gunship went down in flames - also while on a training mission in Albania. But its crewmen escaped with cuts and bruises.

News of the crash came as President Bill Clinton was visiting Nato headquarters in Brussels to discuss the bombing campaign against Serbia.

Fearsome reputation

The BBC's defence correspondent, Mark Laity, says the Apache is designed to fly very low over difficult terrain but that any technical error can lead to a crash.

The BBC's Mark Laity: "Training accidents happen everywhere. This shouldn't cause Clinton any [political] problems"

"Nobody who's involved in Apache operations or in low flying in the RAF would expect to finish a tour without losing friends," he said.

Apaches have a fearsome reputation as tank-busters following the Gulf War, when they were credited with destroying more than 500 Iraqi tanks as well as hundreds of other vehicles.

But concerns have been raised over use their use in Kosovo, because they are relatively slow and therefore vulnerable to surface-to-air missiles and small arms fire.

Two other US aircraft crashed at the weekend. On Saturday, two US warplanes operating over and around Yugoslavia went down. A US Marine Corps AV8B Harrier plane crashed in the Adriatic Sea while on a training mission. The pilot was rescued.

Hours earlier, a US F-16 fighter plane went down over western Serbia.