Three American soldiers were killed 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.
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, also in the east.
The latest deaths take the green-on-blue toll this year to around 33, in some 23 such incidents, according to an AFP tally.
Some of the attacks are claimed by the Taliban, who say they have infiltrated the ranks of Afghan security forces, but many are attributed to cultural differences and antagonism between local and US-led allied forces.
The some 130,000 NATO troops in Afghanistan are training and working alongside Afghan soldiers and police as they take increasing responsibility for the anti-insurgency campaign.
While Western politicians keen to get their troops out of an increasingly unpopular war regularly talk up the ability of the Afghan army and police, the green-on-blue attacks resonate deeply with international forces.
NATO's International Security Assistance Force (ISAF) has said the deaths sapped spirits among its troops.
"Although the incidents are small in number we are aware of the gravity they have as an effect on morale," former ISAF spokesman Brigadier General Carsten Jacobson said earlier this year.
"Every single incident has an out-of-proportion effect on morale and that goes for coalition forces as it goes for Afghan national security forces."
Among the measures being taken, Afghanistan's intelligence services were hiding agents among new recruits at the country's army and police training schools to try to spot potential gunmen, NATO said.
ISAF had also taken several security measures in response to the shootings, including assigning "guardian angels", soldiers who watch over their comrades as they sleep.