Serbian officials said Friday the release of Kosovo's former prime minister from detention in Bulgaria will jeopardize relations between the Balkan neighbors.

A Bulgarian court on Thursday decided to release Agim Ceku from custody, two days after he was detained on an international arrest warrant requested by Serbia for alleged war crimes.

Serbian Foreign Minister Vuk Jeremic said the release of the former ethnic Albanian rebel commander "represents a serious blow for international justice."

Ceku, who was prime minister of Kosovo between 2006 and 2008, is wanted in Belgrade for war crimes allegedly committed during the 1998-1999 fighting in Kosovo. He was taken into custody while entering Bulgaria from Macedonia late Tuesday.

Serbia had asked Bulgaria to extradite him.

In Sofia, Bulgarian Foreign Minister Ivailo Kalfin said Ceku's detention was a "legal and not a political issue."

"The Bulgarian court has considered the legal arguments and made a decision that should have been expected — there are international conventions which provide protection for Mr. Ceku when he comes to Bulgaria," Kalfin said.

"It is a legal issue — not a political one, and I do not want to see it affecting bilateral relations with Serbia in any way whatsoever."