A U.S. Air Force tilt-rotor aircraft crashed in south-eastern Afghanistan, killing three service members and a government contractor, NATO said on Friday.

Other personnel aboard were injured and were taken to a military base for treatment, NATO said.

The CV—22 Osprey went down after dark on Thursday about seven miles (11 kilometers) from Qalat, the capital of Zabul province, NATO said. The cause of the crash was under investigation.

The Osprey takes off and lands like a helicopter, but its engines roll forward in flight, allowing it to fly faster than a standard helicopter.

A Zabul government spokesman, Mohammed Jahn Rasuliyar, confirmed the crash and casualty figure.

A Taliban spokesman had earlier claimed militants shot down the aircraft, part of a pattern of the insurgents making such claims to promote their cause of driving foreign forces from the country.

Meanwhile, a roadside bomb struck a small bus in the Kushki Kuhna district of the western province of Herat, killing three civilians and wounding five, the head of the regional border police, Malam Khan Noorzae, reported.

Roadside bombs are a key Taliban weapon against international forces seeking to suppress militants and stabilize the government of Afghan President Hamid Karzai. Noorzae said a joint NATO—Afghan force had travelled along the same route on Thursday as part of anti-insurgency operations.

NATO also reported that foreign and Afghan troops killed several insurgents during an operation to capture a senior Taliban commander suspected of providing materials used in making roadside bombs. It said two insurgents were captured during the operation on Wednesday in Helmand province’s Kajaki Sofla, but did not say if one was the target of the raid.

On Friday, an international patrol discovered 2,500 pounds (1,130 kilograms) of marijuana seeds in a vacant residential compound in Helmand’s Reg district, NATO said. It said the seeds would be destroyed as part of efforts to cut funding for insurgents from the drug trade.

Helicopters are used extensively by both NATO and the Afghan government forces to transport and supply troops spread across a mountainous country with few roads. Losses have been relatively light, despite insurgent fire and difficult conditions, and most crashes have been accidents caused by maintenance problems or factors such as dust.

Lacking shoulder-fired missiles and other anti-aircraft weapons, the Taliban rely mainly on machine guns and rocket-propelled grenades to target helicopters during landings and takeoffs.

One of the heaviest single-day losses of life for allied forces occurred on June 28, 2005, when 16 U.S. troops died aboard a Special Forces MH—47 Chinook helicopter that was shot down by insurgents.

Thursday’s incident was the first known deadly crash of an Osprey since it entered active service in 2006, although numerous lives were lost in accidents while the aircraft was under development.

The Osprey is the U.S. military’s latest-generation transport aircraft, able to travel twice as fast and three times farther than its predecessor, the Vietnam War-era CH—46 Sea Knight. With room for up to 24 passengers, it comes equipped with sophisticated guidance and missile defence systems.

The original programme, a $40 billion joint venture of Boeing Co. and Textron Inc.’s Bell Helicopter unit, was beset by delays and plagued by design flaws and other problems.

It was nearly cancelled several times due to cost overruns - which pushed the bill to over $100 million per aircraft - and a series of fatal crashes and other incidents. In 2000, a crash in Arizona killed all 19 Marines aboard and a separate crash killed four Marines in Florida.

Critics say the aircraft is particularly vulnerable to ground small-arms fire while its engines are shifting from vertical to horizontal flight. They say that, unlike fixed-wing aircraft, the Osprey can’t glide down to an emergency landing in case of a loss of power and its propellers lack the ability to keep rotating on their own even after the engines fail.

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