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By Guy Delauney & George Wright
BBC News in Belgrade and London
Nato is to deploy an additional 700 troops to Kosovo after 30 Nato peacekeepers and 52 protesters were hurt in clashes on Monday.
Nato Secretary General Jens Stoltenberg said the attacks were "unacceptable" and "must stop".
Police and Nato troops clashed with Serb protesters in north Kosovo, where there has been unrest over the installation of ethnic Albanian mayors.
Protesters had tried to invade a government building in Zvecan.
The crisis dates back to April, when ethnic Serbs boycotted local elections in north Kosovo - allowing ethnic Albanians to take control of local councils with a turnout of less than 4%.
Both the European Union and United States have accused the Kosovan authorities of destabilising the situation in north Kosovo, and warned against any actions that could inflame ethnic tensions there.
Kosovo 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 US and major EU countries - but Serbia, backed by its powerful ally Russia, refuses to do so - as do most ethnic Serbs inside Kosovo.
While ethnic Albanians make up more than 90% of the population in Kosovo as a whole, Serbs form the majority of the population in the northern region.
Monday's violence in Zvecan resulted in 30 troops being hurt, officials said.
Their injuries included fractures and burns from improvised explosive incendiary devices. Three soldiers were also wounded by firearms.
Serbia and Kosovo's leaders have traded accusations over the violent scenes.
Serbian President Aleksandar Vucic said Kosovan PM Albin Kurti "alone is responsible" for the disturbances.
In return, Mr Kurti claimed the protesters in Zvecan were "a bunch of extremists under the direction of official Belgrade".
Mr Stoltenberg of Nato said the military alliance strongly condemned "the unprovoked attacks against KFOR troops", in reference to Nato's peacekeeping force in Kosovo.
But ethnic Serbs in north Kosovo have criticised KFOR for failing to prevent armed Kosovo police from forcing their way into municipal buildings and removing Serbian flags.
Tuesday's announcement from Nato provides a significant boost to KFOR's numbers. The 700 additional troops will join the 3,800 who are already on duty in Kosovo.
An additional reserve battalion has been placed on standby and will be ready to deploy within seven days, if required.
KFOR's mission is to guarantee the safety and freedom of movement of everyone in Kosovo, regardless of their ethnicity.
So the new troops will face considerable expectations from both sides after this week's disturbances.