A European diplomatic intervention has failed to resolve a dangerous row between Serbia and Kosovo over car number plates, with Brussels blaming Pristina for the stalemate.

EU foreign policy chief Josep Borrell hosted Serbian President Aleksandar Vucic and Kosovo Prime Minister Albin Kurti in Brussels amid a row that could trigger a regional crisis.

Afterwards, Mr Borrell said Mr Vucic had been ready to accept an EU compromise proposal on vehicle licensing "that could have avoided this risky situation" - but that Mr Kurti had not.

Pristina has declared that, by next April, around 10,000 Kosovo Serbs with licence plates issued by Serbia must replace them with plates from the Republic of Kosovo.

Mr Borrell told reporters that he would brief EU member states and allies "about the behaviour of the different parties and the lack of respect for international legal obligations".

"And I have to say that, particularly for Kosovo, I know this sends a very negative political signal," he warned.

The underlying source of tension is Kosovo's 2008 declaration of independence, which Serbia does not recognise. Belgrade encourages Kosovo's Serb minority to remain loyal to Serbia.

In the latest development this month, Serbs in northern Kosovo resigned from public institutions in protest over the number plate row.

The dispute sounded alarm bells in the European Union, which has been mediating talks to try to normalise ties, and wants both sides to hold off on provocative gestures.

After yesterday's latest emergency mediation, Mr Vucic insisted that Serbia had been "absolutely constructive" and had agreed to back a text that had been modified "dozens of times".

Serbian President Aleksandar Vucic

"All we did was insist on the fact that the agreements already signed be applied," the Serb leader told TV Pink.

Mr Kurti, however, told reporters that Kosovo was demanding broader talks leading to full normalisation of ties.

"We cannot be irresponsible and not deal with pertinent issues and meet as state leaders that only discuss licence plates and not discuss normalisation of relations," he said.

"That is why we are in the situation we are in."

The United States said it was "disappointed" at the lack of an agreement and called on Mr Kurti "to reach a fair compromise".

"Both Prime Minister Kurti and President Vucic will need to make concessions to ensure that we do not jeopardise decades of hard-won peace in an already fragile region," the State Department said in a statement.

Kosovo Prime Minister Albin Kurti

Washington had urged Kosovo to delay the number plate requirement, arguing that the Western-backed state had been too stubborn and should allow time for EU-led diplomacy.

With the talks at a stalemate, Mr Borrell said he had urged Kosovo to stop implementing its number plate law in "North Kosovo", meaning the main Serb-majority enclave.

And he told Belgrade not to issue new Serb plates to vehicles from Kosovo's cities, arguing that a cooling-off period would allow time and space for diplomacy to resume.

NATO Secretary General Jens Stoltenberg said he had spoken to Mr Borrell about the talks.

"Now is the time for responsibility and pragmatic solutions. Escalation must be avoided," the NATO chief said on social media.

Mr Borrell's spokesman, Peter Stano, stressed the EU was not giving up the search for a solution, but added that "there cannot be any negotiations on normalisation of relations with a threat of violence present".

In August, Brussels brokered a deal to allow free movement between Kosovo and Serbia, after a series of violent incidents.