The Nato-led international force in Afghanistan says an Afghan in army uniform has killed three of its soldiers, the latest in a series of so-called "green-on-blue" attacks.
The attack took place in southern Afghanistan on Tuesday, Nato said, but it did not give the exact location nor the nationalities of those killed.
Some 45 coalition troops have died in more than 30 such attacks this year.
Nato is training Afghan troops ahead of its pullout by the end of next year.
The Taliban has been actively recruiting members of the Afghan security forces, publicly announcing that insider attacks were a central part of their strategy against Nato forces.
A US defence official told Agence France-Presse the latest attack was in Oruzgan province.
The term "green on blue" comes from the colour-coding systems used by the US military.
Currently, approximately 130,000 (blue) Nato troops are fighting insurgents in Afghanistan alongside 350,000 (green) Afghans.
Most of the victims of the attacks have been American.
This month the most senior US military officer, Gen Martin Dempsey, chairman of the US Joint Chiefs of Staff, visited Afghanistan to discuss the rising number of attacks.
He said: "We have an eight-step vetting process that's been in place in earnest for about a year, but we haven't turned the corner on the trend."
However, UN figures released this month did show that civilian casualties had fallen for the first time in eight years.
Those killed or injured fell by 15% in the first half of 2012 compared with the same period last year, the figures showed.