An Afghan police officer has shot and killed three US marines in the third attack on coalition forces by their Afghan counterparts in a week.
The three were killed in the volatile Sangin district of Helmand province, said US military spokeswoman Major Lori Hodge.
A US Defence Department official confirmed that the dead Americans were Marine Special Operations Forces.
The killer targeted the marines after sharing a meal with them and then fled into the desolate darkness of southern Afghanistan.
Sangin's district chief and the Taliban both identified the gunman as Asadullah, a member of the Afghan National Police who was helping the marines train Afghan police.
Taliban spokesman Qari Yousef Ahmadi said the attacker joined the insurgency after the shooting.
The district chief, Mohammad Sharif, said the shooting happened at a police checkpoint after a joint meal and a security meeting. The meal took place before dawn because of Ramadan, the month in which Muslims abstain from food during daylight hours
Four other international troops also died today in two separate attacks, Nato said.
Thirty-one coalition service members have died this year at the hands of Afghan forces or insurgents disguised in Afghan uniforms, according to Nato - a dramatic rise from previous years.
The assaults have cast a shadow of fear and mistrust over US efforts to train Afghan soldiers and police more than 10 years after the US-led invasion to topple the Taliban's hardline Islamist regime for sheltering al Qaida's leadership.