NATO chief Jens Stoltenberg on Sunday called on Kosovo to tone down tensions with Serbia, two days after violent clashes between Kosovan police and protesters 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. He said 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 Kosovo’s 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 by police into their offices.

Protesters threw rocks at the authorities, who responded with tear gas and water cannons to disperse the angry crowds.The situation remained tense on Sunday with heavily armed police in armoured vehicles still guarding the mayors’ offices.