U.S. military officials in Afghanistan say four American soldiers were killed Saturday when their vehicle hit a mine and exploded.

The military says the blast occurred Saturday in the southeastern province of Logar, where U.S. and Afghan soldiers were surveying a potential site for a weapons range.

Officials say they are not certain if the mine had been planted recently or was old unexploded ordnance. Afghanistan is littered with old mines after a quarter century of war.

The deaths came a day after a U.S. Defense Department spokesman, Paul Swiergosz, said American troops will provide transportation and planning support to U.S. and Afghan agencies conducting counter drug operations.

The 17,000 U.S. troops in Afghanistan were previously instructed only to seize or destroy drugs discovered during military operations.