Three American soldiers were killed on Friday when a man in Afghan uniform turned his weapon on 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.
Two Afghan officials told AFP the soldiers were killed by an Afghan police officer who had invited them for a meal at his checkpost in the restive southern province.
"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."
The Sangin district chief, Mohammad Sharif, earlier told AFP that four foreign soldiers had been killed by the checkpost commander after he invited them to a meal.
Breakfast is eaten before sunrise during the holy month of Ramadan, when Muslims fast from then until the evening meal at dusk.
An increasing number of Afghan soldiers and police have turned their weapons against NATO colleagues helping them fight a decade-long insurgency by Taliban Islamists ousted from power in a US-led invasion in 2001.
The attacks are raising questions about the ability of Afghan security forces to take over from NATO combat troops, the bulk of whom are scheduled to withdraw at the end of 2014.