KABUL — A U.S. Air Force Osprey went down in southeastern Afghanistan, the U.S. military said Friday, killing three service members and one civilian contractor in the first crash of the costly tilt-rotor aircraft in a combat zone.

It’s unclear what caused the crash of the U.S. military’s latest generation transport aircraft.

A NATO statement said “numerous other service members” were injured when the aircraft, which takes off and lands like a helicopter, crashed late Thursday seven miles from Qalat, capital of Zabul province about 200 miles southwest of Kabul.

The Air Force version is believed to be used in Afghanistan primarily for resupply missions and for transporting special operations troops hunting Taliban and al-Qaida figures.

The Osprey, which costs nearly $70 million apiece, 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 defense systems.

However, the original development program, 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.

The Osprey was nearly canceled several times due to cost overruns and a series of fatal crashes and other incidents.

In 2000, 19 Marines were killed when an Osprey crashed during a training exercise in Arizona. Another Osprey crash in December of that year killed four Marines in North Carolina.

Critics say the aircraft is particularly vulnerable to small-arms fire while its engines are shifting from vertical to horizontal flight.