* EU summons leaders of Balkan foes back to Brussels
* Bloc to consider on Monday opening accession talks with Serbia
* Serbia balks at UN seat for former province (Updates with Serbian delegation)
By Matt Robinson and Aleksandar Vasovic
BELGRADE, April 18 (Reuters) - The European Union summoned Serbia and Kosovo back to Brussels on Thursday, pressing for an historic accord to settle relations between the Balkan foes and open the door to membership talks with Belgrade.
On the table is an agreement to end the ethnic partition of Kosovo five years since it seceded from Serbia, and potentially clear a path to a seat at the United Nations for the last state to emerge from the ashes of federal Yugoslavia.
An accord would mark a seminal moment in the region's recovery from a decade of war in the 1990s and help unlock Serbia's potential as the largest market in the former Yugoslavia.
With the clock ticking to a Monday decision by the EU on whether to recommend the start of accession talks with Serbia, Belgrade on Wednesday balked at demands by its former province for a seat at the United Nations.
Serbia said it would amount to recognition of a territory it considers the cradle of its nation. Kosovo, where 90 percent of the 1.7 million people are ethnic Albanians, said the issue was non-negotiable.
Both sides headed home, only for Kosovo to announce that its prime minister, former guerrilla commander Hashim Thaci, had turned back from Slovenia's Ljubljana airport at the request of EU foreign policy chief Catherine Ashton.
"Delegations back to Brussels! Dialogue to continue tomorrow!" Kosovo's European Integration Minister Vlora Citaku said on Twitter.
Serbia's leadership was silent, until Prime Minister Ivica Dacic emerged from hours of consultations with his allies to say he too would return. "This represents a huge effort given that we only came back from Brussels today," he told Serbia's Beta news agency.
Any agreement on Friday could mark the culmination of six months of delicate negotiations between Dacic and Thaci, mediated by Ashton.
Kosovo broke away from Belgrade in 1999 after 78 days of NATO air strikes halted the killing and expulsion of ethnic Albanians by Serb forces waging a counter-insurgency campaign under late strongman Slobodan Milosevic.
KEY TO STABILITY
Kosovo declared independence in 2008 and is recognised by more than 90 countries, including the United States and 22 of the EU's 27 members.
Dangling the prospect of membership talks, the EU says Serbia must help to end an ethnic partition of Kosovo between the Albanian majority and a northern pocket of some 50,000 Serbs where Belgrade still has a fragile grip.
The two sides have edged towards a deal on the status of the north, including what autonomous powers it might wield. But negotiations on Wednesday, billed as make-or-break, stalled over Point 14 of the plan. That calls for Serbia to stop obstructing Kosovo's accession to international organisations, implicitly the United Nations.
"Removing that point would undermine the entire agreement," Bekim Collaku, an adviser to Thaci, told Reuters. "What kind of normalisation are we talking about if after this deal Serbia will continue blocking Kosovo on its Euro-Atlantic path?"
Serbia's Dacic said all was not lost, but accused Thaci of trying to scupper the deal.
"Serbia is supposed to let Kosovo be a member of international organisations? Well then let's just write down that we recognise Kosovo as independent," he told reporters. "We couldn't accept that, and we will never accept that."
Settling their relations would go a long way to stabilising the Western Balkans. Kosovo's ethnic partition frequently flares into violence and has frustrated NATO's plans to cut back a peace force that still numbers 6,000 soldiers.
The EU wants to anchor Serbia in accession talks, driving reform and luring investors to a country of over 7 million people. Just as it was the main agitator of the wars that tore Yugoslavia apart, Belgrade today holds the key to regional stability and development.
Ashton is due to report back to EU governments before they make their recommendation on accession talks for Serbia on Monday. That decision would then be finalised in June.
The differences between the two sides are "narrow and very shallow", Ashton said after Wednesday's talks. "We have some hours left." (Additional reporting by Adrian Croft in Brussels, Jaksa Scekic in Belgrade and Fatos Bytyci in Pristina; Writing by Matt Robinson; Editing by Jon Hemming)