KABUL, Afghanistan (AP) - A police officer opened fire on US and Afghan forces inside a police headquarters in eastern Afghanistan on Monday, sparking a firefight that killed two US troops and two other Afghan policemen. The attacker was also killed in the shootout.

The incident in 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 they need to work increasingly close together in order to hand over responsibility as planned next year.

US officials have said they are working with Afghan counterparts on coming up with a solution that will answer President Hamid Karzai's concerns and maintain security in Wardak. The majority of US 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 hold of a machine gun and started firing at the US special operations forces and Afghan police in the police compound in Jalrez district. The assailant killed two Afghan policemen and wounded four, including the district police chief, before he was gunned down.

A coalition military official said two US troops were killed. The official spoke on condition of anonymity to disclose information ahead of a formal statement. A spokesman for US forces in Afghanistan Jamie Graybeal confirmed that a man in an Afghan police uniform turned his weapon on coalition and Afghan forces, and that there were injuries, but declined to give further details or confirm any deaths.

Five Afghan police officers were being held for questioning by the Americans.