Belgrade said it viewed the Kosovan police operation as an attempt to take control of northern parts of Kosovo which border Serbia.
Serbia ordered its troops on full alert Tuesday and Russia accused Kosovo of provocation after a Kosovan police anti-crime operation in a region populated mainly by Serbs led to clashes.
At least 19 people were arrested and a Russian U.N. official detained in the operation. Five police and six Serb civilians were wounded in the fighting, the Kosovan official said. The official was later released Tuesday, the United Nations mission in Kosovo said, after Moscow demanded his release.
The incidents signaled rising tensions in four Serb-majority municipalities in northern Kosovo, parts of which remain largely outside control of Pristina and pledge allegiance to Belgrade.
The United Nations peacekeeping mission in Kosovo appealed for calm.
Belgrade said it viewed the Kosovan police operation as an attempt by Pristina to take control of northern parts of Kosovo which border Serbia.
Serbian President Aleksandar Vucic told parliament that Belgrade would try to "preserve peace and stability" but remained ready to react.
"As the supreme commander, I have placed our army units in full combat readiness, if there is any serious endangering of order and people in the northern Kosovo, they will protect our people," he told deputies.
Kosovan Foreign Minister Behgjet Pacolli said the operation had been carried out against criminal groups in northern Kosovo, where cross-boder smuggling is rampant.
"It was normal work of our police and I believe it's not the last operation in Kosovo. Maybe today or the next days we will keep fighting the organized crime," Pacoli told reporters in the Slovak capital Bratislava.
In Pristina, police chief Rashit Qalaj said a total of 19 Kosovan police officers had been arrested on suspicion of smuggling goods into the country. Eleven were ethnic Serbs, four ethnic Albanians and four were Bosniaks, he told a news conference.
Law enforcement officers had faced "armed resistance," Qalaj said. Five policemen were wounded, one from a gunshot. Six Serb civilians were also hurt, he said.
Majority-Albanian Kosovo declared independence in 2008, almost a decade after NATO air strikes wrested control of the territory away from Belgrade to end a counter-insurgency campaign by Serbian security forces.
But Serbia, which under its constitution considers Kosovo an integral part of its territory, has been blocking Kosovo from membership of international institutions including Interpol and UNESCO. It also still provides financial aid to Serbs in Kosovo.