Kabul - A man wearing an Afghan army uniform killed two NATO troops in southern Afghanistan on Monday, military officials said, the latest in a string of shootings that have undermined trust between allies.

The gunman was killed by NATO troops shortly after he opened fire on a group of foreign troops, the military said in a statement. A military spokesman said officials were investigating whether the man was an Afghan soldier or an infiltrator wearing the uniform. No other details were released.

So-called "green on blue" shootings have become a rising threat this year, following a series of incidents that have created distrust between Afghan forces and their international coalition partners. The most significant was last month's burning of Korans by U.S. troops. The episode sparked violent riots and prompted the Taliban to call on Afghan security forces to open fire on foreign troops.

Since May 2007, at least 51 NATO troops have been killed by Afghan security forces, according to military press releases and statistics provided by the Department of Defense to Congress last month. Nine of those happened after the Koran burning.

In prepared testimony to the House Armed Services Committee, senior defense officials said "the insider threat is an issue of increasing concern," because "it creates distrust between our forces and their Afghan counterparts during a critical juncture in Afghanistan."