The Greek and Turkish leaders of divided Cyprus have headed to Switzerland to launch fresh reunification talks, described by United Nations envoy Espen Barth Eide as the "most complex" round in a series of discussions to solve a decades-old dispute.

Foreign ministers from the three guarantor powers - Britain, Greece and Turkey - will take part in security discussions, alongside the Greek Cypriot and Turkish Cypriot leaders Nicos Anastasiades and Mustafa Akinci, Eide said on Tuesday on the eve of the start of the talks.

"There is an awareness that there is no time like the present," he told a news conference in the Swiss city of Geneva.

"Make no mistake, it is not going to be easy, there is no guarantee of success," he added, calling the talks the best, but not the last, chance for a solution.

The talks in Crans-Montana, outside Geneva, will continue as long as needed, he said, though the UN's current plans envisioned the session lasting until July 7, he added.

Cyprus was 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.

Key issues

Territory, property, governance and power-sharing 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, in reference to the Turkish Cypriot and Greek Cypriot sides respectively.

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.

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

The UK, 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.

Source: News agencies