Israeli investigators were on Tuesday heading to the site of a helicopter crash in mountainous central Romania in which six Israeli airmen and a Romanian national are thought to have died.

The helicopter slammed into the side of a mountain and fell into a valley on late on Monday after becoming separated from its wingman in heavy fog during what an Israeli general described as "routine training" in search and rescue.

The military said it had sent a search and rescue team, crash investigators, medics and rabbis to the scene of the crash, but that they had not yet reached the wreckage or recovered any of the bodies.

"We unfortunately have reason to believe that all seven crew (members) are dead, but at this time no bodies have been recovered," Israeli Brigadier General Relik Shafir told reporters in Jerusalem.

"I think the probability of technical failure is very low," he said, adding that it was "most probably some kind of error or misjudgement that caused the accident."

He insisted, however, that the military had only begun its investigation.

Witnesses quoted by a Romanian television channel said they had seen three helicopters flying above the mountainous region around the city of Brasov.

"Smoke was coming out of one of the helicopters which started losing altitude," one of the witnesses was quoted as saying.

On July 18, the Romanian defence ministry announced an Israeli CH-53 helicopter participating in the same exercise had made a crash landing after "sensing a failure aboard" but then returned safely to its base.

The helicopter, a Sikorsky CH-53, is used primarily for transport and search and rescue missions.

Israel routinely carries out joint exercises with allied air forces in order to train its pilots to operate in unfamiliar terrain, Shafir said.