A senior US military official has told NBC News that the Nato soldier killed by an Afghan Local Police officer in the east of the country was an American.
Earlier two British troops were shot dead by an Afghan soldier.
16 Nato servicemen have been killed by Afghan Security forces so far this year.
Lieutenant Commander Brian Badura from Isaf says they are trying to work out just what happened when two British soldiers were killed by one of their Afghan colleagues.
A Nato serviceman has been shot dead by an Afghan policeman.
The soldier, who was not British, was killed at an Afghan Local Police checkpoint in the east of the country.
Officials say an investigation is underway into the attack, which follows the killing of two British troops by an Afghan soldier.