The prime ministers of Serbia and Kosovo will meet in Brussels on May 21 for follow-up talks on an EU-brokered deal aimed at normalizing their ties, EU foreign policy chief Catherine Ashton's office said Tuesday.

Ashton "invited Prime Minister (Ivaca) Dacic and Prime Minister (Hashim) Thaci for a meeting next week, on 21 May, in Brussels, to discuss the joint steps they need to take for the implementation of the April agreement.

Serbia's Deputy Prime Minister Aleksandar Vucic Sunday urged Serbs living in northern Kosovo to accept an EU-brokered agreement to normalize relations between Serbia and its breakaway former province, while admitting the accord was not ideal.

The 40,000 Serbs make up the majority of residents of northern Kosovo and are largely opposed to the deal, which determines the amount of autonomy afforded to them in Kosovo, which proclaimed independence from Serbia in 2008 -- a move never accepted by Belgrade.

More than 3,000 people protested Friday against an EU-brokered deal to normalize ties between Belgrade and breakaway Kosovo, seen by ultra-nationalists as Serbia's tacit recognition of its former foe's independence.

The protest was called by northern Kosovo Serb political leaders who, along with the ultra-nationalists, have fiercely opposed the implementation of the deal.

An EU-led court in Kosovo on Monday jailed five doctors for organ trafficking at a Pristina clinic in the first such case in the breakaway territory which has already faced allegations of similar crimes during and after its 1998-99 war.

Former Kosovo health secretary Ilir Rrecaj -- who admitted during the trial that he knew that illegal kidney transplants were carried out at the Medicus clinic in Pristina in 2008, but denied covering them up -- was acquitted.

The premiers of Serbia and Kosovo on Friday initialed a historic deal to normalize ties in a move key to the future of the Western Balkans, said EU foreign policy chief Catherine Ashton.

"These negotiations have been concluded," Ashton said. "The text has been initialed by both prime ministers."

Kosovo and Serbia will resume talks in Brussels on Friday following another failure to clinch a deal to normalize ties and ease tension in the Balkans, officials here said.

The Kosovo delegation, which was on its way home after the failed 11th-hour talks on Wednesday, returned to Brussels for the new meeting, the Pristina government said Thursday.

Serbia and Kosovo again failed to clinch a deal to normalize ties and ease tension in the Balkans after lengthy talks brokered by the European Union lasting through Wednesday until Thursday.

"Today has been a very long and intensive day," said EU foreign policy chief Catherine Ashton at the close of negotiations that began early Wednesday but ended after midnight.

Russia on Wednesday vowed to extend economically struggling Serbia a $500-million loan while reasserting support for its ally's continuing refusal to recognize the independence of Kosovo.

Serbian Prime Minister Ivica Dacic won a promise of the quick disbursement of a $300-million (230-million-euro) loan that would help cover Belgrade's budget shortfall.

Serbia rejected on Monday a Brussels-brokered proposal on easing tensions with breakaway Kosovo, seen as a key condition for EU integration talks to begin, but pledged to continue negotiations with Pristina.

"The government of Serbia cannot accept principles verbally presented to its negotiating team in Brussels, since they do not guarantee full security and protection of human rights to the Serb people in Kosovo," Serbian Prime Minister Ivica Dacic told reporters.

A former top Kosovo health ministry official on trial for organ trafficking on Friday admitted that illegal kidney transplants were carried out at a Pristina clinic, but denied covering them up.

Ilir Rrecaj told an EU-led panel of judges that the "transplants happened, but there was no licence for (them)" at the Medicus clinic.