Tensions flared in the Balkans on Tuesday after police in Kosovo raided Serb-populated areas.
Dozens of people were arrested, including a Serbian and a Russian working for the United Nations, officials said. Some 90 percent of people living in Kosovo are ethnically Albanian, with ethnic Serbs making up a tiny minority.
In response, neighboring Serbia ordered its troops near the border to be on high alert, and said it would be ready to protect its people if necessary.
Authorities said Tuesday's operation targeted organized crime and smuggling, which is rampant in the area following a 100 percent tariff on all Serbian goods was imposed by Kosovo last year.
Kosovo police said in a statement they were met with resistance during the raid, with locals setting up barricades and two officers being shot and injured.
Kosovo broke away from Serbia following a U.S.-led NATO bombing campaign in 1999 and declared independence in 2008.
A decade on, there are still 650 American troops in Kosovo among a total NATO presence totaling 3,500. NATO's KFOR peacekeeping force said Tuesday that it was "ready to intervene" if requested, and called for both countries to "deal with the disputes peacefully and responsibly, without any use of force or violence."
Serbian President Aleksandar Vucic told his troops near the border to be on "combat alert" and ready to protect the country's ethnic diaspora if necessary.
"Serbia will try to preserve peace and stability, but will be fully ready to protect our people at the shortest notice," Vucic told Parliament, according to The Associated Press.
Two of those arrested were one Russian and one Serbian national who worked at the United Nations mission in Kosovo, the mission confirmed to NBC News in a statement. Both Moscow, Belgrade and the U.N. have called for their release.
Alexander Chepurin, the Russian ambassador to Belgrade, said his country was "revolted by the provocation."
Zahir Tanin, the head of the U.N. mission, called for their immediate release, warning that any harm to United Nations staff will be met with the highest degree of diplomatic and international legal responses."
Kosovo President Hashim Thaci called Serbia's response "populist" and claimed that no one ethnic group was targeted in the raids. He said that one of those arrested was a Russian citizen was "camouflaged under a diplomatic veil to hamper the police operation."