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Serbia's president has placed the army on alert and moved units close to the Kosovo border, after clashes between police and Kosovo's Serb minority.
Ten people were injured in the violence after residents gathered outside state buildings in the Serb majority border town of Zvecan.
Police used tear gas to disperse the protesters, and gunshots and explosions could be heard in videos posted online.
Clashes began after police moved to install new ethnic Albanian mayors.
Kosovo's Serb minority - which accounts for about 5% of the country's 1.8 million population - boycotted April's local elections in four northern municipalities, allowing ethnic Albanians to take control of the councils.
Belgrade backed the boycott - resulting in a turnout of just 3.47% - which was sparked after the Serb community demanded the establishment of a promised association of Serbian municipalities.
The Serbian minority says the association would work on education, healthcare, land planning and economic development, but ethnic Albanians fear it could allow the formation of a pro-Serbian statelet.
After the newly elected mayors attempted to take office on Friday, Serb residents - who rallied to the sound of a community alarm used to warn of a police presence - sought to block them.
Kosovan police then moved to escort the politicians past protesters, leading to clashes.
Kosovan officials acknowledged their officers had increased their presence in the area "to assist mayors of the northern communes of Zvecan, Leposavic and Zubin Potok to exert their right of work at the official objects".
Local officers in Zvecan said five officers were injured and at least four vehicles were damaged during the unrest, with one car destroyed after it was set on fire.
Serbia's Defence Minister Milos Vucevic said President Aleksander Vucic had ordered an "urgent movement [of troops] to the Kosovo border".
"It is clear that the terror against the Serb community in Kosovo is happening," he said.
US Secretary of State Antony Blinken criticised the use of force by Kosovan government, saying that it had taken action against the advice of the US and EU allies.
Mr Blinken said the move had "sharply and unnecessarily escalated tensions in the region" and had served to undermine efforts to "normalise relations between Kosovo and Serbia".
Kosovo unilaterally declared independence from Serbia in February 2008, after years of strained relations between its Serb and mainly Albanian inhabitants.
It has been recognised by the United States and major European Union countries, but Serbia, backed by its powerful ally Russia, refuses to do so, as do most ethnic Serbs inside Kosovo.
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