Three international soldiers were killed on Saturday in what NATO said was an insider attack in eastern Afghanistan.
A NATO spokesperson said a man wearing an Afghan army uniform turned his weapon on soldiers in Paktika province, close to the Pakistan border.
Some reports have said the three soldiers killed were Americans.
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"We can confirm three US service members were killed today when an individual wearing an ANA uniform turned his weapon against the ISAF service members in eastern Afghanistan," an International Security Assistance Force (ISAF) press officer said in a statement.
"ISAF and Afghan officials are assessing the incident and more information will be released as appropriate. It is ISAF policy to defer casualty identification procedures to the relevant national authorities."
Government officials also said an Italian soldier died in a separate attack in the western province of Farah.
BBC News correspondent David Loyn said there had been a spate of insider attacks on international troops last year, but that the last one had occurred more than a month ago. He also noted that coalition forces train, mentor and fight alongside Afghan soldiers, which makes them vulnerable.