Updated

Tensions soared in the Balkans a day before Kosovo's parliament is set to approve the formation of an army, with Serbia warning Thursday that the move would threaten peace in the war-scarred region.

Kosovo's parliament is expected Friday to pass legislation turning an existing 4,000-strong security force into an expanded, lightly armed army.

After talks with the head the U.N. mission in Kosovo, Serbian President Aleksandar Vucic described Kosovo's decision as a "new provocative and one-sided measure" that raises Serbia's concerns for the future of Serbs in Kosovo.

Vucic said Serbia will "do its best to preserve peace and stability," but warned the "situation will be considerably worsened" if Kosovo goes ahead with the decision on Friday.

Serbia alleges that the army's main purpose is to ethnically cleanse Kosovo's Serbian-dominated north, a claim strongly denied by Pristina.

Kosovo President Hashim Thaci, wearing a military jacket, visited army barracks near the capital Pristina to assure "all the communities in Kosovo, especially the Serb one" that the future army "will continue to serve them without any ethnic distinction."

Serbia lost control of Kosovo after a 1998-99 war and NATO airstrikes that ended Belgrade's bloody crackdown against Kosovo Albanians. In 2008, Kosovo declared independence. The US and most of the West recognize it, but Serbia and its allies Russia and China do not.

Adding to tensions among Serbs ahead of the Friday vote, Kosovo's Security Forces held exercises in the south and NATO-led peacekeepers deployed a convoy of combat vehicles in the north of Kosovo.

The deployment of peacekeepers was seen as a provocation by Kosovo Serbs, but the so-called KFOR mission said it was a routine exercise.

"It's just a regular training, exercise. KFOR troops training to keep readiness to be rapidly deployed all over Kosovo as it is foreseen by our mandate," said the mission's spokesman, Vincenzo Grasso.

He also sought to downplay the importance of Friday's vote in parliament.

"Whatever will be decided tomorrow will not change the situation in one day," he said.

The Kosovo security force commander, Lt. Gen. Rrahman Rama, said that there should be no fear of Kosovo's new army which would not be "a threatening force."

"Our main purpose is to build a force that will be multiethnic, comprehensive and that will serve all citizens without distinction," Rama told The Associated Press.

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Llazar Semini from Tirana, Albania, and Jovana Gec, from Belgrade, Serbia, contributed.