Two U.S. troops in Afghanistan were killed after an individual wearing an Afghan National Police uniform turned his weapon on them, officials said Thursday.

The attack took place in Khas Uruzgan district. Officials are investigating the incident.

It was the latest in a string of attacks from inside the Afghan Army and police force that are threatening to undermine both the partnership with international troops -- which have been the target of many attacks -- and the morale of Afghan forces, who have suffered equally heavy casualties from such strikes.

So far this year, at least 52 foreign troops -- about half of them Americans -- have been killed in insider attacks.

The Afghan government has not provided statistics on the number of its forces killed in insider attacks. However, U.S. military statistics obtained by The Associated Press show at least 53 members of the Afghan security forces had been killed in such strikes by the end of August.

The Associated Press contributed to this report.