American among two dead in Afghan insider attack, official says September 30, 2012 -- Updated 1113 GMT (1913 HKT) NEW: American among two killed in insider attack in Afghanistan, ISAF official says The clash occurred at a checkpoint in Afghanistan An unknown number of Afghan soldiers also died Kabul, Afghanistan (CNN) -- An American service member was killed in a clash between Afghan army forces and international troops, an International Security Assistance Force official in Kabul said Sunday. The official spoke on condition of anonymity because individual countries officially announce the nationalities of ISAF troops who are killed. The Saturday incident, the latest apparent insider attack by members of Afghan security forces, also led to the death of a civilian contractor, authorities said. An unknown number of Afghan soldiers also died in the clash at a checkpoint in eastern Afghanistan, said Abdul Qayoom Baqizoi, a provincial police chief. It is too early to tell what happened, the police chief said. A joint investigation between local and international authorities is under way, officials said. Insider attacks by Afghan soldiers and police officers - or militants wearing uniforms - have been on the rise for months. More than 50 coalition troops have died in so-called "green-on-blue" attacks this year, raising concerns about the stability of Afghan security forces ahead of planned NATO withdrawals in 2014. "Green-on-blue" refers to a color coding system used by the military, in which blue refers to international troops and green refers to allied Afghan forces. NATO's ISAF troops are training Afghan soldiers and police to maintain security in the country ahead of the planned end of allied combat operations. This month, U.S. troops in Afghanistan were ordered to halt joint operations with local security forces in some areas on account of such attacks.