NATO chief Jens Stoltenberg told Kosovo and Serbia to "calm down" on Friday, following a string of disputes that have sent bilateral tensions rising to their highest level in years.

On a visit to Kosovo, the ANATO secretary general urged all parties to "show restraint and participate in a constructive way" in European Union-brokered talks between the two sides, which were at war in the late 1990s.

"My main message to everyone is to calm down, not use the rhetoric that can increase tensions," he told reporters.

"It is vital that the agreements reached in this dialogue are implemented."

Serbia does not recognise the sovereignty of its former province Kosovo, which declared independence in 2008 a decade after its ethnic Albanian insurgents fought Serbian forces under late strongman Slobodan Milosevic.

The troops withdrew from the territory following a 11-week bombing campaign by NATO against Serbia.

In 2011 Kosovo and Serbia began the EU-led talks to improve their damaged ties, but relations deteriorated again in recent weeks.

Last month Belgrade sent a train towards Kosovo, painted in the Serbian national colours and covered with the statement "Kosovo is Serbia" in multiple languages.

The move was lambasted as a "deliberate provocation" by Kosovo's President Hashim Thaci, who suggested it was part of a plan to annexe the Serb-populated north of his country.

Serbian Prime Minister Aleksandar Vucic eventually halted the train before it crossed the border, saying he took the decision to "avoid a conflict and save lives" based on fears the train would be attacked.

Kosovo was also angered by the arrest in France last month of its former prime minister Ramush Haradinaj on suspicion of war crimes, under an international arrest warrant issued by Serbia.

Leaders of the two Balkan countries have since met in Brussels and agreed to try and ease tensions.