The killing of a Kosovo police officer and an ensuing gun battle at a monastery in a village close to the Serbian border on Sunday marked one of the gravest escalations in the former breakaway province in years.

NATO chief Jens Stoltenberg said that on Thursday he had "authorised additional forces to address the current situation".

His statement did not give details, but Britain's defence ministry said it had made a battalion of between 500 and 650 troops available to the force, known as KFOR.

The battalion recently arrived in the region for long-planned exercises, the British ministry said.

"We will always take all necessary actions to maintain a safe and secure environment and freedom of movement for all people living in Kosovo," Stoltenberg said.

In the north of Kosovo, where the Serb minority is concentrated, KFOR has decided to "increase its presence and activity", added a NATO official who requested anonymity.

The official explained that KFOR had already increased its presence in Kosovo in May, deploying around 500 Turkish troops, who were subsequently replaced by Bulgarian and Greek soldiers.

He added that KFOR was prepared to make "further adjustments" if necessary to enable it to fulfil its peacekeeping mandate.

The White House said Friday it had detected a large build-up of Serbian forces on the Kosovo border and urged Belgrade to withdraw them to avoid destabilising the situation.