Kabul ? An Afghan policeman on patrol with U.S. soldiers opened fire on the Americans, killing two of them before fleeing, officials said Saturday, raising questions about discipline in the ranks of the Afghan forces and possible infiltration by insurgents.

Training and operating jointly with Afghan police and soldiers is key to the U.S. strategy of dealing with the spreading Taliban-led insurgency and, ultimately, allowing international forces to leave Afghanistan. But Afghan forces have periodically turned their guns on international soldiers.