The sixth round of Cyprus peace talks ended in disagreement today threatening to pro-long differences between Turkey and the United States.

A joint communique issued after a week of negotiations between Cyprus' Greek and Turkish communities said it was "not possible to bridge the considerable differences between the two sides."

The United States has made resumption of military aid to Turkey conditional on progress toward a Cyprus settlement.

The arms embargo, imposed by congress following Turkey's seizure in 1974 of 38 per cent of Cyprus, has seriously strained relations between Ankara and Washington. Turkey, the bulwark of NATO's southeast defense, has threatened to review its contribution to the alliance if U.S. aid is not soon restored.

The Greek-Cypriots blamed the Turkish-Cypriots for lack of progress at the Vienna talks. The Turkish-Cypriots denied this and said the Greek-Cypriots were purposefully painting a black picture in order to prolong the U.S. arms embargo on Turkey.

"The talks were very constructive." Turkish-Cypriot negotiator Umit Onan said. "Although there are still differences, we feel the talks have given both sides the opportunity to go into the problems in depth and get a better understanding of each others views."

Greek-Cypriot negotiator Tassos Papadopoulos said the negotiations "did not come up to expectations. If there was common ground," he said, "it was so significant I did not notice it."

He charged the Turkish-Cypriots had blocked progress by failing to make counterproposals to Greek-Cypiort demands that they return more than half of the territory they now control on Cyprus. "Mere comments or rejection [WORD ILLEGIBLE] proposals are not meaningful negotiations he said.

The Turkish- [WORD ILLEGIBLE] negotiator blamed the United Nations , which sponsored the meeting, for misleading the Greek-Cypriots over Turkish-Cypriot intentions.

He said agreed guidelines for the talks obliged the Turkish Cypriots only to make proposals on the future political makeup of the island, and not make counterproposals to the Greek-Cypriot territorial demands.

"The Turkish-Cypriot side gave their views on the Greek-Cypriot territorial proposals and explained why they considered them unrealistic and totally unacceptable," Onan said.

The proposals called for the return to the Greek-Cypriots of about half of the territory Turkey holds, leaving the Turkish Cypriots with 18 per cent - roughly equivalent to their population - of the island.

Onan said the map presented by the Greek-Cypriots, which would transfer the Turkish-controlled port of Famagusta, the citrus-growing region of Morphu and the copper mines of Lefka to the Greeks - would leave the Turks without economic resources and their security vulnerable.

Proposals on the political future of the island sumitted by the Turkish-Cypriots called fro a "federation by evolution."

"We should start cautiously and realistically . . . with two separate administrations, functioning in two separate regions . . . with only limited powers and functions given to the federal government," Onan said.

He said that with passage of time and the restoration of mutual trust between the two communities, which have fought each other three times in the past 13 years ago, a strong federal republic would develop.

Onan likened the transformation to the evolution of the United States from a confederation to a federation.

The Greek-Cypriots counter-proposed a strong federal government responsible for foreign policy, defence and economy, with control of the island's air and sea ports.

Onan said the proposals were identical to ones submitted by the Greek-Cypriots in April, 1976, and already rejected by the Turks.

Onan and Papodopoulos said the current round of negotiations, resumed March 31 after a 13-month breakdown, would be continued in Nicosia in May.