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A police officer opened fire on U.S. and Afghan forces at a police headquarters in eastern Afghanistan on Monday, sparking a firefight that killed two U.S. troops and two other Afghan policemen. The attacker was also killed in the shootout, officials said.

In a second incident, outside Kabul, U.S. troops fired on a truck approaching their military convoy, killing two Afghan men inside.

The shooting in the eastern Wardak province was the latest in a series of insider attacks against coalition and Afghan forces that have threatened to undermine their alliance at a time when co-operation would aid the planned handover of security responsibility to local forces next year.

The attack also comes a day after the expiration of the Afghan president's deadline for U.S. special forces to withdraw from the province.

U.S. officials have said that they are working with Afghan counterparts to answer President Hamid Karzai's concerns and maintain security in Wardak. Most of the U.S. troops in Wardak are special operations forces.

In Monday's attack, an Afghan police officer stood up in the back of a police pickup truck, grabbed a machine-gun and started firing at the U.S. special operations forces and Afghan policemen in the police compound in Jalrez district, said the province's Deputy Police Chief Abdul Razaq Koraishi.

The assailant killed two Afghan policemen and wounded four, including the district police chief, before he was gunned down, Koraishi said.