A US Air Force Osprey went down in southeastern Afghanistan, the US 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 US military's latest generation transport aircraft — beset for years by cost overruns and design flaws.A NATO statement said "numerous other service members" were injured when the aircraft, which takes off and lands like a helicopter, crashed late Thursday 11 kilometers from Qalat, capital of Zabul province about 300 kilometers southwest of Kabul.A Taliban spokesman said militants shot down the aircraft, but the insurgents often make exaggerated claims. NATO said the cause was still under investigation. A Pentagon spokesman, Marine Maj. Shawn Turner, said it was the first time that an Osprey, which cost nearly $70 million apiece, has crashed during operations in a war zone.After years of tests and modification, the Ospreys went into service in the Air Force and Marine Corps in 2006. They were used by the Marines in Iraq starting in late 2007, mostly for transporting troops and supplies in Anbar province. But they were not truly tested in combat there. The worst of the fighting in Anbar had already passed because many insurgents there had switched sides.A Marine Osprey flew then-candidate Barack Obama from Iraq to Amman, Jordan, during his visit to the Middle East in July 2008.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.Aircraft are used extensively in Afghanistan by both NATO and the Afghan government forces to transport and supply troops spread across this mountainous country, where roads are few and primitive.Losses have been relatively light because unlike Iraqi insurgents, the Taliban lack shoulder-fired missiles and other anti-aircraft weapons, relying mostly on machine guns and rocket-propelled grenades to fire at aircraft during landings and takeoffs.Nevertheless, one of the heaviest single-day losses of life for allied forces in Afghanistan occurred on June 28, 2005, when 16 US troops died aboard a Special Forces MH-47 Chinook helicopter that was shot down by insurgents.The Osprey is the US 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.Former Vice President Dick Cheney tried to kill the program in 1989 when he was defense secretary, saying the aircraft were unnecessary. But the Marines persuaded Congress to authorize the funds.Critics say the aircraft is particularly vulnerable to small-arms fire while its engines are shifting from vertical to horizontal flight. Unlike fixed-wing aircraft, the Osprey cannot glide down to an emergency landing in case of a loss of power. Unlike helicopters, its propellers lack the capability to keep rotating on their own even after the engines fail.But the Osprey's supporters say the aircraft makes up for vulnerabilities by its speed, saying it can rise quickly to altitudes safe from small-arms fire. The Osprey's engines roll forward in flight, allowing it to fly faster than a standard helicopter.Also Friday, a roadside bomb struck a small bus in the western province of Herat, killing three civilians and wounding five, the head of the regional border police, Malam Khan Noorzae, said. Noorzae said a joint NATO-Afghan force had traveled along the same route the day before as part of anti-insurgency operations.Elsewhere, flights resumed Friday at the US Manas air base in the Central Asian nation of Kyrgyzstan after being halted two days before during an uprising against the government. Manas is a key support center for the international military campaign against the Taliban in Afghanistan, but opposition figures in the past have said they wanted to close the base."Obviously Manas is a very important air base for our operations inAfghanistan," President Obama's national security adviser, Gen. JimJones, told reporters Friday accompanying Obama back to Washington fromPrague. "But all that we'll just have to wait and see how it plays outbecause it's just too early."Jones also indicated that tensions had eased between the administrationand Afghan President Hamid Karzai, who infuriated the White House withallegations that the UN and the international community interfered inlast year's fraud-tarnished presidential election in Afghanistan."We have gotten through this period," Jones said.The Obama administration is counting on Karzai's support during anupcoming military operation to clear the Taliban from Kandahar, thebiggest city in southern Afghanistan and the Islamist movement'sbirthplace. Karzai was born in a village near Kandahar and his halfbrother is a major political figure there.