Serbian troops have reportedly been placed on "combat alert" early this week after police in the breakaway Republic of Kosovo, which has never been recognized by Serbia following its 2008 independence declaration (though backed by the US and 100 nations), raided Serb-populated regions to arrest close to two dozen as part of an "an anti-smuggling mission".

Belgrade has accused the ethnic Albanian-majority Kosovo police force of targeting Serbs in the region who still pledge allegiance to the Serbian state. The Serbian president charged Kosovo police with "bursting" into the region with armored vehicles and wounding "unarmed Serbs".

According to Reuters:

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 fighting, Kosovan official said.

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 UN mission in Kosovo has appealed for calm on all sides and said it was "concerned" over the detention of two of its staff members by the Kosovo police, one of which is of Russian nationality. 

Russia also expressed outrage, with Alexander Chepurin, the Russian ambassador to Belgrade, saying on Twitter that Moscow is "revolted by the provocation". Serbian President Aleksandar Vucic declared in statements that Serbia's military was prepared to defend Serbs in Kosovo.