A member of a European Union policing mission in Kosovo was killed and two others were injured in a roadside ambush in the volatile northern region of the country, where hundreds of American soldiers are among NATO troops watching over a fragile peace.

The shooting drew immediate international condemnation.

EU foreign minister Catherine Ashton called for a swift response to the attack, in which a pair of vehicles carrying officers with the mission came under gunfire in the northern town of Zvecan. A Lithuanian policeman died and two other officers were injured, according to an EU release. NATO Secretary-General Anders Fogh Rasmussen called the attack “unacceptable.”

The vehicles carried six police officers returning south from a customs gate on Kosovo’s northern border with Serbia.

The EULEX mission works with police and judiciary officials in the small Balkan nation, which declared independence from Serbia in 2008. Tensions between ethnic Serbs in Kosovo’s north, who refuse to recognize the Pristina government, and ethnic Albanians to the south, occasionally flare into violence.

Troops with NATO’s Kosovo Force came under attack several times in 2011 and 2012 as they tried to remove barriers erected by ethnic Serbs to isolate the north from the rest of the country. An agreement between leaders of the two nations earlier this year suggested relations were smoothing, although extremists are still active on both sides.

Close to 700 U.S. soldiers remain in the country as part of the NATO peacekeeping mission.

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