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".

Members of a Kosovo special police unit near the village of Cabra, northwestern Kosovo, during police raids, via the AP.

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.

However, Kosovo's prime minister Ramush Haradinaj, on the other hand, defended the operation, which involved heavily armed SWAT police: "I confirm that the operation is about law enforcement and nothing else. I call Serbs in the north to stay calm and respect the law," the prime minister said.

Though there are conflicting details of precisely how the shootout began, statements in Reuters suggest a large-scale armed confrontation in a busy Serbian enclave:

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.

Other Kosovo officials described it as a fight against "organized crime" amid accusations that cross-border smuggling has seen a significant uptick recently. The Kosovo government has enacted a 100 tariff on all Serbian goods following last year's controversial maneuver by Serbia to have Pristina blocked from joining Interpol.

On Wednesday Kosovo’s prime minister resisted EU calls to revoke or suspend the tariff on Serb goods, saying it would remain in effect until Serbia formally recognized Kosovo's borders.

In 1999 US and NATO intervention controversially wrested Kosovo - Serbia's medieval heartland - from Belgrade's control, bombing Yugoslavia in a multi-month air campaign that directly killed somewhere between 400 and 500 plus Serbian civilians, possibly many more, along with decimating infrastructure like bridges, hospitals, and government buildings.

In total, the 1998-1999 war for Kosovo claimed more than 13,000 lives according to most estimates. Tensions have remained high ever since, spilling over into internecine ethnic violence and destruction of religious buildings in the years that followed up to today.