"I welcome recent steps taken by Pristina to lower tension, including reducing the number of special police in the north, and plans to facilitate new municipal elections," Stoltenberg said after meeting Kosovar President Vjosa Osmani-Sadriu.

"But it is essential to avoid further escalation. So I urge all parties to avoid inflammatory rhetoric and to act with restraint and in line with their commitments."

He said the situation was "an opportunity for Kosovo to demonstrate that it is a responsible actor, working constructively for the benefit of Euro-Atlantic security".

Unrest rocked Serb-majority areas in the north of the territory in May, including a riot by ethnic Serbs that Stoltenberg said left 93 NATO peacekeepers injured, some seriously.

Northern Kosovo has remained riven by divisions and periods of unrest since Pristina declared independence from Serbia in 2008.

Tensions skyrocketed after Kosovo's leadership installed ethnic Albanian mayors in four Serb-majority municipalities.

Kosovo is overwhelmingly populated by ethnic Albanians, but in the northern stretches of the territory near the border with Serbia, ethnic Serbs remain the majority in several municipalities.

KFOR is the country's top security institution and it bolstered its force level to 4,500 troops in the wake of the violent clashes in May.

The peacekeeping force has been stationed in Kosovo since the end of the 1998-1999 war between ethnic Albanian separatist guerrillas and Serbian forces.

The EU -- which has been conducting a years-long push to normalise ties between Kosovo and Serbia -- has invited leaders from the two sides for a new round of talks next week.