Suspected insider attack kills U.S. soldier, contractor in Afghanistan ISLAMABAD, Pakistan - A U.S. soldier and an American civilian contractor were shot to death this weekend in what NATO officials Sunday said was a suspected insider attack, the latest in a disturbing surge of so-called "green-on-blue" killings that have threatened collaboration between Afghan and NATO forces ahead of the planned withdrawal of U.S. troops in 2014. The attack, which occurred late Saturday afternoon in the eastern province of Wardak, comes just days after top U.S. military officials said joint operations between U.S. and Afghan forces were resuming following a temporary halt imposed by the U.S. because of the rising number of insider attacks. Gen. Abdul Qayum Baqizoi, Wardak's police chief, said gunfire broke out between NATO and Afghan army troops at a checkpoint in the province's Sayedabad district. Three other NATO soldiers were injured in the gunfight. Three of the seven Afghan army soldiers deployed at the checkpoint were killed, Baqizoi said. A "misunderstanding" led to the incident," Baqizoi added, but he would not elaborate. A short statement issued by NATO described the incident as "a suspected insider attack." A joint NATO-Afghan security force investigation into the attack was underway, according to the statement. More than 50 U.S. and coalition troops have been killed in insider attacks this year, a significant jump from 2011, when 35 NATO soldiers were killed in such attacks. U.S. officials have said that the Taliban insurgency accounts for about a quarter of the attacks, either through infiltration of Afghan security forces or influence on Afghan troops. Disputes stemming from cultural differences are also often cited as a cause for the attacks. Roughly 15% of all NATO troop deaths in Afghanistan are the result of "insider" shootings. More than 1,950 U.S. troops have died in Afghanistan during the 11-year conflict, according to icasualties.org, a web site that tracks NATO troop deaths in Afghanistan. Insider attacks are becoming a major impediment to Washington's exit strategy in Afghanistan, partly because cooperation between U.S. and Afghan security forces is needed as NATO continues to hand over more responsibility for securing the country to Afghan troops. That cooperation includes joint NATO-Afghan troop operations to track down insurgent commanders and fighters, as well as a continued step-up in NATO training of Afghan soldiers and police. Earlier this month, NATO responded to the rise in insider attacks by temporarily suspending joint operations with Afghan security forces, unless those operations were approved by a high-ranking regional commander. The restrictions remain in place, but last week officials in Washington said cooperation on joint operations had resumed. Alex Rodriguez and Aimal Yaqubi. Staff writer Alex Rodriguez reported from Islamabad, Pakistan, and special correspondent Aimal Yaqubi reported from Kabul, Afghanistan. Photo: A tattoo on the back of a U.S. Army sergeant is seen through his torn shirt after a foot patrol in Afghanistan's Kandahar province. The full tattoo reads, "Sacrifice. Without fear there is no courage. Credit: Julie Jacobson / AP Photo