The CH46 Sea Knight transport helicopter and AH-1 Cobra attack helicopter collided over Courthouse Bay, five miles north of the beach where British and US Marines had begun landing at midnight.
The aircraft crashed in flames at 2am, their wreckage landing in a marshy, wooded area, making identification difficult. No British personnel were involved.
As the troops began landing - the climax of the exercise - dozens of helicopters flew overhead, showing red safety lights, but more reliant than usual on night vision equipment. At one point on Thursday a thousand aircraft were in the air.
Although the Sea Knight is approaching the end of its useful life, Pentagon sources said yesterday they did not think mechanical failure was to blame.
The US authorities have launched an investigation, which is expected to take several months.
Flying operations were halted immediately after the crash but resumed after dawn. The US Marines - whose headquarters are at nearby Camp Lejeune - were devastated by the news, but said they were carrying on with the exercise.
The Secretary of State for Defence, Michael Portillo, who landed with one of the waves of British troops at 8.30am yesterday, said the British fleet was flying flags at half-mast out of respect for the US casualties.
"It just goes to show that, even on an exercise, servicemen take risks," Mr Portillo said.
"Of course they'll be a shadow, but none the less the exercise will be of great value to save life in the future."
Exercise "Purple Star" is the biggest British amphibious operation since the Falklands and the biggest joint US and British exercise of this type since the Allies' Normandy landings in 1944.
A total of 38,200 US personnel and 15,600 British are involved. The US Air Force is providing 171 aircraft, the RAF 56: the US Navy 26 ships and the Royal Navy 27.
But most of the aircraft are from the US Marines' Second Marine Expeditionary Force which has 30 flying squadrons, totalling more than 400 aircraft. The main body of this force - 40,000 Marines - is based at Camp Lejeune and the air component at two Marine Corps air stations, New River and Cherry Point. The helicopters involved in Friday morning's crash were from 266 Marine Aviation Squadron based at New River.
The US authorities declined to release the names of the dead until their families had been informed.Reuse content