The U.N.'s special adviser on Cyprus, Espen Barth Eide | Philippe Desmazes/AFP via Getty Images Cyprus reunification at an impasse as UN fails to find compromise Onus now firmly back on Turkish and Greek Cypriot leaders.

Long-running negotiations to reunify Cyprus hit a dead-end on Friday, when the United Nations said it was unable to broker a plan for discussing the final and toughest issues on the table.

At the request of Greek Cypriot leader Nicos Anastasiades and Turkish Cypriot leader Mustafa Akıncı, the U.N.'s special adviser on Cyprus, Espen Barth Eide, spent the last nine days shuttling between the two sides in an attempt to agree on the framework for a second international conference in Geneva.

"Unfortunately, despite serious efforts to overcome their differences regarding the modalities for meeting in Geneva, the leaders were unable to find common ground," Barth Eide said in a statement Friday. "Without a prospect for common ground, there is no basis for continuing this shuttle diplomacy."

The sticking point was over the procedure of the talks, rather than the substance.

Anastasiades, president of Cyprus' internationally recognized government, wants the Geneva summit to start with the most controversial issue in the talks — the powers granted to Greece, Turkey and the U.K. to guarantee Cyprus' security and territorial integrity — and then move on to outstanding issues such as the boundary between the two states that would make up a unified Cyprus. Akıncı, president of the self-declared Turkish Cypriot state, wants to discuss all the issues at the same time, out of fear that he would otherwise lose a bargaining chip.

Barth Eide said he will ask U.N. Secretary-General António Guterres for advice on how to proceed. However, the U.N. made clear that it's now up to Anastasiades and Akıncı to push ahead with the negotiations — if they choose to do so.

Although the Greek and Turkish Cypriot sides could still agree on how to move forward, supporters of reunification worry the window of opportunity is closing fast.

"The talks have not collapsed, the process remains leader-led," said Aleem Siddique, spokesman for the U.N.'s Peacekeeping Force in Cyprus. "We need to hear from the leaders how they want to take the process forward."

Barth Eide's announcement will come as a huge disappointment for many who had hoped Anastasiades and Akıncı would succeed in ending the island's 43-year division. The two sides have been negotiating a reunification deal for about two years. The U.N. facilitates the talks, but the leaders set the tone, pace and schedule.

Anastasiades and Akıncı held the first-ever international conference on Cyprus at U.N. headquarters in Geneva in January with Greece, Turkey, the U.K., the European Union and the U.N. The three guarantor powers will have to sign off on any agreement to change the current system, set up after the U.K. granted Cyprus independence in 1960. Crucially, that would include a plan for drawing down, if not doing away with, the roughly 40,000 Turkish troops stationed in the northern Turkish Cypriot side of the island.

But the January summit ended in disappointment, following disagreement over the security and guarantees system. Greece and Greek Cypriots want to do away with it entirely, while Turkey and Turkish Cypriots want to phase it out slowly over several years. The talks then stalled as the two sides blamed one another for the impasse.

Anastasiades and Akıncı quickly pointed fingers at one another following Barth Eide's statement on Friday afternoon, while stressing that they're both committed to holding another Geneva conference.

Akıncı argued that Anastasiades' push to address the security and guarantees issue first would "fully eliminate the principle of discussing all issues interrelatedly," which they agreed to do in February 2014. "We obviously do not consent to a negotiations process under preconditions,” he said in a statement.

Anastasiades said he learned of the Turkish Cypriot side's decision not to accept the Geneva compromise or Barth Eide's invitation to a leaders' meeting this Monday "with great sadness," and that he remains ready to take part in a conference with "genuine and effective negotiation."

Although the Greek and Turkish Cypriot sides could still agree on how to move forward, supporters of reunification worry the window of opportunity is closing fast.

Tensions are already flaring over companies' plans to begin drilling for oil and gas off the island's coast under permits issued by the Greek Cypriot government, despite protests from Turkey and Turkish Cypriots.

In an additional distraction, Anastasiades will soon have to turn his attention to campaigning for re-election in February.