Updated

Cyprus' rival leaders hope to achieve a breakthrough reunification agreement when they meet in Crans-Montana, Switzerland, later this month, a United Nations envoy said Friday.

Espen Barth Eide said the U.N. is preparing for the possibility that the summit starting June 28 could "last for weeks."

He said if there's success, more work will be needed to finalize a peace accord that would be put to a vote in both the Greek and Turkish Cypriot communities.

"This is really a historic opportunity," Eide said after talks with the island's Greek Cypriot President Nicos Anastasiades.

"It's not getting easier just because we're going to Switzerland, but there is a determination and a willingness that I can register now that we go with that ambition."

The summit is seen as the culmination of more than two years of negotiations between Anastasiades and the leader of the breakaway Turkish Cypriots, Mustafa Akinci. It will focus on a security structure after the island is reunified as federation and will include Cyprus' "guarantors" — Greece, Turkey and Britain.

Other outstanding issues, like how much territory will make up each side's federal zones, will be discussed in parallel but in a separate room.

An agreement on security will to a large extent determine whether there will be an overarching peace deal. The issue is extremely difficult and that's why it hasn't been tackled until now.

It revolves around the 35,000-plus troops that Turkey kept in place in the island's breakaway, Turkish Cypriot north when it invaded in 1974 following a coup by supporters of union with Greece. Greek Cypriots see those troops as a threat and want them gone; Turkish Cypriots see them as their sole protection and want them to stay.

Eide said a document now being readied that will break down the core elements of the remaining issues will help steer negotiations. It will also incorporate ideas to help overcome difficulties, Eide said, but didn't elaborate.