The North Atlantic Treaty Organization warned of mounting instability in the Balkans after lawmakers in Kosovo voted to establish an army, prompting outrage in neighboring Serbia, which disputes the country’s independence.

Kosovo has been governed as an international protectorate, with NATO handling its security, since it declared independence in 2008, nine years after U.S.-led NATO forces stopped Serbia’s military crackdown on its breakaway province.

The two countries have in recent years achieved progress in negotiating a lasting arrangement to put their differences aside under a series of talks hosted by the European Union, culminating in a proposal this year to swap territory. But the momentum has since been lost, and Kosovo’s push to establish its armed forces is seen as a major setback for the reconciliation process.

The vote also further eroded the common front Europe and the U.S. had shared on Kosovo, with NATO condemning the vote and Washington welcoming it.

Serbian Foreign Minister Ivica Dacic told the state-owned press agency Tanjug that he would call for an emergency meeting of the United Nations Security Council. Mr. Dacic wasn’t immediately available to comment.