EU and Serbia say Eulex murder was political

Serbian Prime Minister Ivica Dacic said in Belgrade: "It's not a shot at Eulex [the EU police mission in Kosovo], but at the future of the Serb people in Kosovo, Serbia and the future of its citizens."

Aleksandar Vucic, Serbia's deputy PM, added: "Those who believe that such crimes can get away unpunished should know that they will never have a safe haven in the territory of Serbia."

Dacic himself on 19 April in Brussels agreed a plan with Kosovo leader Hashim Thaci to "normalise" relations with Kosovo, which split from Serbia in the 1990s Balkan wars.

The plan includes holding municipal elections in north Kosovo - a self-ruled, ethnic-Serb enclave - in November before putting the region under Pristina's control.

Serbia's implementation of the deal is being watched by EU countries before they set a date for starting EU-Serbia accession talks.

Ethnic Serbs in north Kosovo, who operate paramilitary forces, have held protests against the Dacic-Thaci plan.

But the region is also home to violent organised crime groups who make a fortune from cross-border smuggling.

The incident occurred at 07.30am local time near the town of Zvecan.

Eulex chief Bernd Borchardt told press that a "person or persons" opened fire from an "ambush" on two Eulex cars on a routine customs patrol.

The deceased, Audrius Senavicius, a 35-year-old father of two sons, later died in a hospital in the town of Mitrovica from multiple gunshot wounds. He had arrived in Kosovo in August 2012.

Another man in the same car, a Czech-origin Eulex officer, is in hospital being treated for suspected internal injuries.

Borchardt thanked Serbia for its offer to help bring the killer to justice.

"If someone tries to escape, we would gladly take their [Serbia's] help, I mean to escape across the boundary line [between north Kosovo and Serbia]," he said.

Amid messages of condolence from EU foreign ministers and from Nato, Borchardt added: "I am shocked, very deeply shocked, at the callous nature of the murder."

For her part, EU foreign affairs chief Catherine Ashton said in a statement that she is also "deeply saddened." But she added that the EU will "press on with its work in the context of the EU-facilitated dialogue" on Kosovo-Serbia relations.

Violent clashes between ethnic Serbs and Eulex, Kosovo police or Nato personnel are nothing new in north Kosovo.

In August 2011, a sniper shot and killed a Kosovar policeman, Enver Zymberi, a 22-year-old father of four.

But the Eulex death is its first one since the mission began work in 2008.

Around 40 people have died while serving in EU civilian and military crisis operations abroad in the past 15 years, most of them in the Western Balkans or in Africa.