Serbia and Kosovo failed Wednesday to find common ground on how to defuse longstanding tensions at EU-sponsored talks, officials said.

"The gap between the two sides is very narrow but deep," said EU foreign affairs head Catherine Ashton, who chaired the eighth round of talks in Brussels aimed at normalizing ties between Serbia and breakaway territory Kosovo.

Serbian President Tomislav Nikolic on Monday cast doubt on whether a final agreement with breakaway Kosovo is attainable, a day ahead of highly anticipated EU-sponsored talks with the Pristina leadership in Brussels.

"Intractable statements from Pristina put us before a fait accompli and are not encouraging," Nikolic told reporters ahead of Tuesday's meeting.

The EU said Wednesday that significant progress has been made in talks with Serbia and Kosovo aimed at defusing tensions in one of Europe's last trouble-spots, having demanded concrete results from the two parties.

EU foreign affairs head Catherine Ashton said a fifth series of talks she chaired with Serbian Prime Minister Ivica Dacic and Kosovo Prime Minister Hashim Thaci had gone well, with all sides pleased at the outcome and another meeting set for March 4.

Serbia named its key negotiator for Kosovo as its first diplomatic representative to Pristina since the breakaway territory unilaterally declared independence in 2008, an official said Monday.

The move, seen as a further step in improving relations between Belgrade and Pristina under EU-mediated dialogue, came just a few days after Kosovo named a career diplomat, Lulzim Peci, as its representative to Serbia.

Serbian President Tomislav Nikolic will meet his Kosovo counterpart Atifete Jahjaga in Brussels next week, in a first such meeting since breakaway Kosovo declared independence in 2008, officials said Friday.

"President Nikolic will meet Atifete Jahjaga in Brussels on Wednesday, February 6," said the message on Nikolic's official Twitter account.

Kosovo named a career diplomat for its first diplomatic representative to Serbia since Pristina unilaterally declared independence in 2008, as a further step in establishing relations with Belgrade under EU-mediated dialogue, an official said Tuesday.

The move came after Kosovo and Serbia agreed during Brussels-mediated talks to exchange liaison officers whose main goal would be to improve and step up communication between Belgrade and Pristina.

Two border crossings between Serbia and its former province Kosovo, which Belgrade does not recognize, opened under joint management for the first time Monday, following European Union mediation.

The two crossings in Merdare and Jarinje, which had previously been functioning as informal checkpoints between Serbia and northern Kosovo, were officially opened for both business and individual travelers.

The premiers of Serbia and Kosovo hold a third round of EU-brokered talks over a working dinner Tuesday amid hopes of a deal to ease tension along their flashpoint border.

Talks between Serbian Prime Minister Ivica Dacic and his Kosovo counterpart Hashim Thaci will be hosted, as previously, by European Union foreign policy head Catherine Ashton.

U.S. Secretary of State Hillary Clinton on Wednesday said that Kosovo's unilaterally declared independence, fiercely opposed by Serbia, was not debatable.

"(The United States) will oppose any discussion of territorial changes or reopening Kosovo’s independent status. These matters are not up for discussion," she said, reiterating Washington's strong support for Kosovo's ethnic Albanian authorities on a visit to Pristina.

U.S. Secretary of State Hillary Clinton begins a five-nation Balkans tour on Tuesday, seeking to push regional leaders to stick to existing agreements and enact the reforms needed to join the EU and NATO.

Clinton, who will be joined by European Union foreign policy chief Catherine Ashton during her first stop in Sarajevo, is expected to call on Bosnia to speed up reforms in a country still deeply divided along ethnic lines 17 years after the 1992-1995 war there.