Rescue operations are under way across Afghanistan, after three helicopters crashed in a series of pre-dawn sorties, leaving at least four US troops dead and two others seriously injured.

Two helicopters collided in mid-air, in the South of the country, while a third went down under heavy fire in the West, moments after extracting soldiers from a daring night raid against one of the region’s most wanted drug-smugglers.

US officials said that hostile fire was not involved in the mid-air collision, but they are still investigating the cause of the second crash.

The soldiers injured in the collision were evacuated to military hospitals, but a rescue operation to recover casualties from the second crash was still ongoing, officials said.

Elite US troops had launched an airborne assault against a compound in western Afghanistan, believed to “harbour insurgents” linked to Afghanistan’s multi-billion dollar opium trade.

“During the operation, insurgent forces engaged the joint force and more than a dozen enemy fighters were killed in the ensuing fire fight,” a military spokesman said.

“As the joint force was departing the area, one helicopter went down due to unconfirmed reasons. Military casualties are reported and a recovery operation is under way.”

Details of the units and the types of helicopters involved have been withheld until the rescue operations are complete.

Both southern and western Afghanistan have seen a surge of US troops.

A spokesman for the Taleban claimed responsibility. Qari Yousuf Ahmadi said: “There was fighting in the bazaar between foreign troops and the Taleban. During the fighting the Taleban shot down a foreign helicopter.”

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