File photo of US soldiers on patrol with Afghanistan National Army (ANA) soldiers in Helmand Province in June 2012. Three American soldiers were killed Friday when a man in an Afghan uniform turned his weapon against them, the US military said, the latest in a series of so-called green-on-blue attacks.

Three American soldiers were killed Friday when a man in an Afghan uniform turned his weapon against them, the US military said, in the third so-called green-on-blue attack in less than a week.

A spokeswoman said "an individual in an Afghan uniform fired on US Forces Afghanistan services members in Sangin district of Helmand province. There were three US Forces Afghanistan fatalities.

"We're investigating to determine the facts," she said, without giving details of whether the shooter had been killed, captured or escaped.

The Sangin district chief in the southern province, Mohammad Sharif, told AFP that four foreign special forces soldiers had been killed by an Afghan policeman after he had invited them to dinner.

"Last night, a police checkpost commander invited four foreign special forces soldiers to a dinner party at the checkpost. He later killed them and ran away," Sharif said.

Apart from Americans, NATO soldiers from other countries, including Britain, serve in the restive province.

An increasing number of Afghan soldiers and police have turned their weapons against NATO colleagues helping them fight a decade-long Taliban insurgency.

On Tuesday, an American soldier was killed in the east when two men in Afghan army uniform opened fire, and on Thursday an Afghan soldier was killed after turning his weapon on NATO troops.