But Isaf said an ongoing investigation had not yet determined the cause of the crash.

Al Jazeera's James Bays, reporting from Kabul, said: "The Osprey is an aircraft that is seen very often in Afghanistan."

"The military like this aircraft because it has the speed of a plane and the agility of a helicopter."

The CV-22 conducts long-range infiltration and resupply missions for US Forces. It employs tilt-rotor technology that allows it to take off and land as a helicopter. While in the air, the engines can roll forward, allowing the aircraft to fly faster than a standard helicopter.

"This aircraft was equipped with extra fuel tanks as it was on a special mission," Bays said.

Previous crashes

Technical problems were blamed for a similar Nato helicopter crash in Zabul, which injured 14 people, on March 29.

This crash raises the number of foreign troops who have died during the war in Afghanistan this year to 151.

Last month, a helicopter carrying Turkish soldiers crash landed in Wardak.

Meanwhile, three Taliban fighters were killed and two injured by US troops in the northern Afghan province of Kunduz, Mohammad Omar, governor of the province, said on Friday.

Taliban commander Mullah Gai was among those killed, Omar said.

Two other fighters were killed and three Afghan policemen were injured during a clash in the neighbouring province of Takhar on Thursday, provincial police chief Sher Ahmad said.