An Afghan man wearing a uniform has shot and killed three US soldiers in the southern province of Helmand, a NATO spokesperson has announced.
The incident took place early on Friday morning, and details were still being gathered, Major Lori Hodge, the spokesperson for the the military alliance in Afghanistan, said.
NATO was looking into whether the attacker was a member of the Afghan military or security forces.
Afghan officials said the men were all special forces members and were killed while attending a meeting in Sangin district.
Mohammed Sharif, the Sangin district chief, put the toll higher, saying 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.
Other Afghan officials confirmed that account.
"Asadullah, the police checkpost commander, invited four foreign special forces soldiers to a [Ramadan] breakfast at 2:30 am in Sangin district," a senior security officer in the province said, requesting anonymity.
"He later opened fire on the special forces soldiers, killing three and wounding another, and he managed to run away."
Qari Yousuf Ahmadi, a Taliban spokesman, speaking to the AFP news agency by phone said, "Asadullah joined the mujahideen ranks after the killing".
Friday's incident, is the third so-called green-on-blue assault - in which Afghan forces turn their weapons against NATO allies - in four days. The deaths take the toll of such attacks to around 33 this year in some 23 such incidents, according to a tally by the AFP news agency.
NATO, puts the count of such attacks at 24 since January, causing 28 deaths.
Last year, there were 21 such attacks, in which 35 people were killed.
In a separate incident, three US soldiers and a US aid worker were killed on Thursday in the eastern province of Kunar, after a suicide attack.