GENEVA (Reuters) - Talks on the long-running Cyprus conflict are the best chance, but maybe not the last chance, to get an agreement, U.N. envoy Espen Barth Eide on Tuesday on the eve of negotiations in a Swiss mountain resort.

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Foreign ministers from the three guarantor powers - Britain, Greece and Turkey - are due to stay this week to take part in security discussions, alongside the Greek Cypriot and Turkish Cypriot leaders Nicos Anastasiades and Mustafa Akinci, Eide said.

“There is an awareness that there is no time like the present,” he told a news conference in Geneva. “Make no mistake, it is not going to be easy, there is no guarantee of success,” he added, calling it the “most complex” round in the series of talks.

The talks in Crans-Montana will continue as long as needed, he said, though the United Nations’s current plans envisioned the session lasting until July 7, he added.

Divided in a 1974 Turkish invasion that was triggered by an abortive coup by supporters of union with Greece, mediators say the two sides have come closer than ever but have left the hard parts until last.

Territory, property, governance and powersharing were “make or break” issues, Eide said. The two sides have also been diametrically opposed on security, especially the presence of about 30,000 Turkish troops in northern Cyprus.

“One side puts more emphasis on the security of community, because of a sense of being the smaller community numerically, and the other side puts more emphasis on security of state,” he said.

The goal is a peace deal uniting Cyprus under a federal umbrella and which could also define the future of Europe’s relations with Turkey, a key player in the conflict.

Eide said there had been a significant exchange of ideas since a previous round of talks three months ago.

Britain, which has military bases on the island, is offering to give up about half of its land as part of a deal. Foreign Secretary Boris Johnson said in a statement that with political will, creativity and flexibility, a deal could be done.

“It’s an exceptional opportunity for both communities to find a lasting solution for Cyprus, which will bring huge benefits to the whole island and the region,” he said.