BELGRADE, Serbia - Serbia put its troops on full alert Tuesday after heavily armed Kosovo police entered Serb-dominated northern Kosovo, firing tear gas and arresting about two dozen people in what they called an anti-organized crime operation.

It was the latest flare-up in long-simmering tensions between Serbia and its former province, which declared independence from Belgrade in 2008 after a bloody 1998-99 war that ended only with NATO intervention. Serbs make up 90% of the population in northern Kosovo and they want to remain part of Serbia, not an independent Kosovo. Action by Kosovo special police there is rare and always triggers Serb anger.

Serbian President Aleksandar Vucic said Kosovo police arrested 23 people, including Serbs, Bosnians and a Russian, after “bursting” into several northern villages and the town of Mitrovica with armoured vehicles. Vucic said he had seen video of the police firing “live ammunition” over the heads of unarmed Serbs, and said the operation was designed to intimidate minority Serbs in Kosovo, whose population is mostly ethnic Albanians.

Vucic said he has ordered soldiers near the border to be on “combat alert” to protect Serbs if tensions escalate.

“Serbia will try to preserve peace and stability, but will be fully ready to protect our people at the shortest notice,” Vucic told parliament.

He later said that the Kosovo policemen withdrew from northern Kosovo. But in a sign of lingering tensions, Serb troops and armoured vehicles were seen leaving their barracks in central Serbia and headed toward the Kosovo border. An air force MiG-29 fighter jet made a low pass near the border with Kosovo, according to the state Tanjug news agency.

Any Serbian armed incursion into Kosovo would mean a direct clash with NATO-led peacekeepers there. Lately, Serbia has been making frequent military threats against Kosovo as its political and military alliance with Russia grows stronger despite a proclaimed European Union membership goal.

Kosovo police said five police officers and six civilians were injured during an operation against organized crime and corruption, and 19 police officers and a number of other people were arrested.

The U.N. mission in Kosovo said those detained included two of its staff, one of them Russian. It said both employees were hospitalized for injuries, and it called for all parties to help restore calm and security.

Later, the mission said the Russian citizen was released and Tanjug reported he was taken to a Serb hospital in northern Mitrovica with injuries to the head and face.

Russia called Kosovo’s actions a “provocation” and demanded the immediate release of the Russian U.N. employee.

Kosovo President Hashim Thaci said earlier that the Russian “was camouflaged under a diplomatic veil to hamper the police operation.”

Kosovo Prime Minister Ramush Haradinaj urged the ethnic Serb minority to remain calm and support the police.

“Those involved in illegal activities will go behind bars,” he wrote on his Facebook page, insisting that the police operation was not targeting people from specific ethnicities.

Kosovo’s foreign minister said his country’s authorities are set to carry out more police actions like Tuesday’s operation that sent tensions with Serbia soaring.

“I believe it’s not the last operation in Kosovo. ... Maybe today, maybe tomorrow or in next days. We are fighting organized crime,” said the minister, Behgjet Pacolli.

The spokesman for the NATO peacekeeping mission, Col. Vincenzo Grasso, said the force is monitoring the situation and co-ordinating with authorities.

“Because of the political sensitivity of the moment, Commander KFOR invites all the parties to deal with the disputes peacefully and responsibly, without any use of force or violence. People should stay calm, they have nothing to fear,” the mission said in a statement.

Serbia and allies Russia and China do not recognize Kosovo’s 2008 declaration of independence, while the U.S. and more than 100 other countries do. The lingering dispute has stalled both countries’ efforts to become EU members.

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The two sides had been taking part in an EU-facilitated dialogue, but Serbia walked away in November after Kosovo slapped a 100% tax on Bosnian and Serbian imports, saying it will be lifted only when the two countries recognize Kosovo’s sovereignty.

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Associated Press writers Jovana Gec in Belgrade and Llazar Semini in Tirana, Albania, contributed.