"We strongly condemn the unprovoked attacks against KFOR troops in northern Kosovo, which led to 30 peacekeepers being wounded. Such attacks are unacceptable and must stop," Stoltenberg said in Oslo.

"Violence sets back Kosovo and the entire region" and puts the Balkan countries' hopes of joining NATO "at risk", he said.

"Both Pristina and Belgrade must take concrete steps to de-escalate the situation, refrain from further irresponsible behaviour and engage in the EU-facilitated dialogue, which is the only way to lasting peace," Stoltenberg said.

NATO had already announced that it was deploying more forces to northern Kosovo where the situation remained tense Tuesday, with ethnic Serbs gathered outside the town hall that a crowd had tried to storm Monday.

Kosovo police had repelled them with tear gas, before the NATO-led peacekeepers intervened.

The soldiers at first tried to separate protesters from the police, but later tried to disperse the crowd using shields and batons.

Several protesters responded by hurling rocks, bottles and Molotov cocktails at the soldiers.

Kosovo's ethnic Serb minority boycotted local elections in the north in April, allowing ethnic Albanians to take control of the local councils despite a tiny turnout of less than 3.5 percent.