A person wearing an Afghan police uniform shot dead two Nato service members inside a compound in northern Afghanistan, the military alliance said.
Nato stressed that it was still investigating the incident in Faryab province. Authorities said the shooter fled the scene. The military coalition did not provide further details.
It was not immediately clear if the shooter was an Afghan police officer. There have been incidents of both Afghan security forces turning on their Western counterparts and of insurgents donning uniforms to infiltrate bases and attack from the inside.
Officials in Faryab could not immediately be reached for comment.
The shooting comes as protests erupted in Afghanistan again today against a Florida pastor's burning of the Quran, making four straight days of demonstrations — some deadly — against the destruction of Islam's holy book.
The Quran burning appears to have inflamed a simmering anti-foreigner sentiment in the country, where anger about civilian casualties and international contractors making fortunes off the long-running conflict have worn down the welcome for Western forces over more than nine years of fighting.
Turncoat attacks by Afghan police and soldiers have appeared to increase over the past 12 months as Nato and Afghan forces work more closely together. In some cases, such shootings have been a result of arguments that turned violent; in others, the Taliban has claimed that Afghan shooters were sleeper agents.
In the last such incident in January, an Afghan soldier approached two Italian soldiers who were cleaning their weapons and shot both of them dead before escaping from the base.
One of the deadliest such shootings occurred in November when an Afghan border police officer opened fire on Nato troops during a training mission in eastern Nangarhar province, killing six Nato service members before he was shot dead.