Tensions in the region had been building since the elections last month.

In a televised statement early Tuesday, Mr. Vucic blamed Kosovo’s prime minister, Albin Kurti, for fueling hostilities. He also criticized NATO’s peacekeeping mission, saying it had failed to protect the Serbian population and was enabling the “illegal and forceful takeover” of the majority-Serbian municipalities by the Kosovan government.

Mr. Kurti, Kosovo’s prime minister, applauded the NATO forces, saying they had been trying to curb the “violent extremism” in the streets. “In a democracy there is no place for fascist violence,” he said in a statement on Twitter. “Citizens of all ethnicities have a right to full & unencumbered service of their elected officials,” he added.

What’s behind the conflict?

The latest escalation is part of a dispute over the status of Kosovo, which declared its independence 15 years ago, almost a decade after NATO’s 78-day bombing campaign in 1999 that drove Serb forces, then engaged in brutal mistreatment of ethnic Albanians, from Kosovo.

While an independent Kosovo has been recognized by the United States and many European countries, Serbia — as well as its key allies, Russia and China — still refuses to recognize Kosovo’s independence. It has called the split a violation of U.N. resolution 1244 that dates back to 1999 and the end of the Kosovo war.