A man dressed as an Afghan police officer opened fire on NATO troops Monday, killing six Americans.

The NATO forces were taking part in a training exercise with Afghan security teams in the Eastern part of the country, near the border with Pakistan.

Insurgents there routinely put on police uniforms to attack government installations. The Taliban have claimed responsibility for Monday's attack.

"Today he found this opportunity, and he killed six invaders," Taliban spokesman Zabiullah Mujahid said.

The latest shooting highlights the hazards facing the U.S. in its push to expand Afghanistan's police and armed forces.

NATO troops plan to turn over security control to Afghan forces by 2014.