Serbia and Montenegro has strongly condemned the nomination of former rebel leader Agim Ceku as the new prime minister of Kosovo.
Mr Ceku's appointment could further strain ties with Serbia
A government spokesman in Belgrade said Mr Ceku should be put on trial for war crimes, not elected to office.
Kosovo's President Fatmir Sejdiu called on Mr Ceku to form a government after the resignation of Bajram Kosumi.
Mr Ceku commanded the pro-independence Kosovo Liberation Army (KLA), which fought against Serbian forces.
Before that, he fought with the Croatian army against the Serbs during the break-up of Yugoslavia in the 1990s.
Mr Ceku, an ethnic Albanian, is currently the head of the Kosovo Protection Corps, a civil emergency force.
Serbia has issued an arrest warrant for him, accusing him of crimes against Serbs in both Kosovo and Croatia.
"The Serbian government finds it absolutely unacceptable that a man who should be tried for war crimes could be elected to any political office," said a government spokesman in Belgrade.
His nomination for the post of prime minister still requires endorsement from the UN-administered province's parliament.
The former Prime Minister, Bajram Kosumi, resigned on Wednesday amid widespread criticism of his administration and pressure from within the governing coalition.
The change comes less than two weeks after UN-sponsored talks on the future of Kosovo began in the Austrian capital, Vienna. Diplomats hope the talks may lead to a deal on Kosovo's status this year.
The BBC's Balkans analyst Gabriel Partos says Mr Ceku is very popular among Kosovo Albanians and has been seen as a dynamic leader. But his appointment might make Belgrade more reluctant to engage in the negotiating process on Kosovo's status, he says.
Kosovo Albanians, who make up the majority, want independence for the province, still legally part of Serbia and Montenegro. But Serbia is concerned about the rights of the Serb minority.