Three people were killed and 13 others were injured when an explosion rocked Bagram Airfield this morning, the largest U.S. military base in Afghanistan and located next to the ancient city of Bagram, more than 30 miles north of Kabul.<br/><br/>It is not clear what caused the explosion or the nationality of the victims, the district governor of Bagram, Haji Shokoor said. No militant group has so far claimed responsibility for the blast. <br/><br/>The explosion underscores a worsening security situation nearly two years after NATO formally ended its combat operations in Afghanistan.