BELGRADE, Serbia (AP) — Serbia put its troops on full alert Tuesday after heavily armed police in northern Kosovo entered Serb-populated Mitrovica, firing tear gas and arresting nearly two dozen people.

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. Ninety percent of population in northern Kosovo are Serbs who don’t want to be part of independent Kosovo. Action by Kosovo special police there is rare and always triggers Serb anger.

Serbian President Aleksandar Vucic said Kosovo police arrested 23 Serbs and Bosnians after “bursting” into the northern area with armored vehicles. He 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.

Serbian state TV reported movements of Serb troops stationed near the border. Any Serbian armed incursion into Kosovo would mean a direct clash with NATO-led peacekeepers there.

Kosovo’s prime minister, Ramush Haradinaj, confirmed on Twitter that police had carried out “an anti-smuggling and organized crime operation.” President Hashim Thaci called on 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.

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

Serbia, and its allies Russia and China, do not recognize Kosovo’s 2008 declaration of independence. The United States and most other countries do. The dispute has stalled both countries’ efforts to become members of the European Union.

The two sides had been participating 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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By DUSAN STOJANOVIC Associated Press

AP writers Jovana Gec in Belgrade and Llazar Semini in Tirana, Albania, contributed to this report.

Kosovo police special unit members secure the area near the village of Cabra, north western Kosovo, during an ongoing police operation on Tuesday, May 28, 2019. (AP Photo/Visar Kryeziu)

Aleksandar Vucic, the president of Serbia, center, prepares to speak during a session of Serbia’s parliament in Belgrade, Serbia Monday, May 27, 2019. Vucic addressed the Serb parlamentarians during a session devoted to the situation in Kosovo, which declared independence from Serbia in 2008. (AP Photo/Marko Drobnjakovic)

Kosovo police special unit members secure the area near the village of Cabra, north western Kosovo, during an ongoing police operation on Tuesday, May 28, 2019. (AP Photo/Visar Kryeziu)

Kosovo police special unit members secure the area near the village of Cabra, north western Kosovo, during an ongoing police operation on Tuesday, May 28, 2019. (AP Photo/Visar Kryeziu)

Kosovo police special unit members secure the area near the village of Cabra, north western Kosovo, during an ongoing police operation on Tuesday, May 28, 2019. A Kosovo police operation against organized crime in the north, where most of the ethnic Serb minority lives, has sparked tension, and Serbia ordered its troops to full alert. (AP Photo/Visar Kryeziu)

Kosovo police special unit members stop vehicles near the village of Cabra, north western Kosovo, during an ongoing police operation on Tuesday, May 28, 2019. A Kosovo police operation against organized crime in the north, where most of the ethnic Serb minority lives, has sparked tension, and Serbia ordered its troops to full alert. (AP Photo/Visar Kryeziu)

Kosovo police special unit members secure the area near the village of Cabra, north western Kosovo, during an ongoing police operation on Tuesday, May 28, 2019. A Kosovo police operation against organized crime in the north, where most of the ethnic Serb minority lives, has sparked tension, and Serbia ordered its troops to full alert. (AP Photo/Visar Kryeziu)

Kosovo police special unit members secure the area near the village of Cabra, north western Kosovo, during an ongoing police operation on Tuesday, May 28, 2019. A Kosovo police operation against organized crime in the north, where most of the ethnic Serb minority lives, has sparked tension, and Serbia ordered its troops to full alert. (AP Photo/Visar Kryeziu)

Kosovo police special unit members secure the area near the village of Cabra, north western Kosovo, during an ongoing police operation on Tuesday, May 28, 2019. A Kosovo police operation against organized crime in the north, where most of the ethnic Serb minority lives, has sparked tension, and Serbia ordered its troops to full alert. (AP Photo/Visar Kryeziu)