By Sabine Siebold
BRUSSELS (Reuters) -NATO chief Jens Stoltenberg called on Kosovo to tone down tensions with Serbia on Sunday, two days after clashes between Kosovan police and protesters who are opposed to Albanian mayors taking office in ethnic Serbian areas.
Stoltenberg, the transatlantic military alliance's Norwegian secretary-general, said he had spoken to European Union foreign policy chief Josep Borrell about Kosovo and that Pristina and Belgrade must engage in the EU-led dialogue.
"Pristina must de-escalate & not take unilateral, destabilising steps," Stoltenberg said in a tweet.
Serbs, who form the majority of the population in Kosovo's northern region, do not accept its 2008 declaration of independence from Serbia and still see Belgrade as their capital more than two decades after the war ended in 1999.
Ethnic Albanians make up more than 90% of the population in Kosovo as a whole.
Serbs refused to take part in local elections in April and Albanian candidates won all four municipalities with a 3.5% turnout.
Local Serbs, backed by Belgrade, said they will not accept the mayors and that they do not represent them.
On Friday, three out of four mayors were escorted into their offices by police, who were pelted with rocks and responded with tear gas and water cannons to disperse the protesters.
Heavily armed police in armoured vehicles were still guarding the mayors' offices on Sunday.
A joint statement from the embassies of the United States, Italy, France, Germany and Britain, known as the Quint group, and the EU office in Pristina warned Kosovo against any other measures to force access to the municipality buildings.
"We strongly caution all parties against other threats or actions which could impact on a safe and secure environment, including freedom of movement, and that could inflame tensions or promote conflict," Quint and the EU said.
"New unilateral actions will negatively impact relations with the Quint countries and the EU."
The United States, Britain and the EU are Kosovo's main backers as the country is still not a United Nations member due to objections from Serbia, Russia, China and others.