UN Secretary General Kofi Annan has been putting the final touches to the latest draft of his plan for the reunification of Cyprus.
Diplomats want a deal before Cyprus joins the EU
Mr Annan, in Switzerland for vital talks on Cyprus's future, is expected to present his revised version to the
Greek and Turkish prime ministers.
They will join talks in the next 24 hours to help find a way forward.
EU membership will in effect apply only in the Greek part of Cyprus if no deal is reached by the end of the month.
Mr Annan's blueprint on Cyprus is apparently more than 1,000 pages long and its terms are still being adjusted to take into account some of the points raised during weeks of discussion by the four principal parties, says the BBC's David Bamford in Lucerne.
Hammering out the deal are Greek Foreign Minister Petros Moliviatis, his Turkish counterpart Abdullah Gul, Greek Cypriot President Tassos Papadopoulos and Turkish Cypriot chief negotiator Mehmet Ali Talat.
Veteran Turkish Cypriot leader Rauf Denktash has refused to attend the talks.
Mr Annan has sought to find a middle position between the many conflicting demands of the two communities on Cyprus.
His blueprint envisages a power-sharing arrangement giving the Greek and Turkish communities large degrees of autonomy over domestic affairs.
Mr Annan is also focusing on how many of the 180,000 Greek Cypriots can move back into the northern third of Cyprus from where they fled after the Turkish military invasion 30 years ago.
But whatever is agreed must conform to EU principles about the free movement of citizens, our correspondent says.
The talks, held under United Nations auspices, are taking place in the mountain retreat of Buergenstock in the mountains above Lake Lucerne and are subject to a news blackout.
The four parties have been asked to reach agreement by 31 March.
If they fail to do so, Mr Annan will "fill in the gaps", and the proposals will be put to referendums in the Greek and Turkish parts of the Mediterranean island on 20 April.
If either side rejects the deal, the island will not be reunited by the time it accedes to the EU on 1 May, and effectively only the Greek Cypriots will join.
In 1983, the Turkish-held area declared itself the Turkish Republic of Northern Cyprus, and is recognised internationally only by Turkey.