As more time passes, efforts toward finding a mutually agreeable solution to the Cyprus issue are growing more complicated, United Nations chief Antonio Guterres said Monday.

In a closed session of the U.N. Security Council, Guterres said the trust that a negotiated solution will be reached was dwindling and that both sides should maintain dialogue to keep the peoples’ hope alive in the absence of full-fledged negotiations.

Guterres said that internal political developments on both sides are creating additional problems and that the two parties have grown more dissociated in their attitudes.

The island of Cyprus has been mired in a decadeslong struggle between the Turkish and Greek Cypriots, despite a series of diplomatic efforts by the U.N. to achieve a comprehensive settlement. The island has been divided since 1964 when ethnic attacks forced the Turkish Cypriots to withdraw into enclaves for their safety. In 1974, a Greek Cypriot coup aiming at Greece's annexation led to Turkey's military intervention as a guarantor power. The Turkish Republic of Northern Cyprus (TRNC) was founded in 1983. Turkey is the only country that officially recognizes the TRNC as a separate and independent state.

The Greek Cypriot administration, backed by Greece, became a member of the European Union in 2004, although in a referendum that year most Greek Cypriots rejected a U.N. settlement plan that envisaged a reunited Cyprus joining the EU.

While Greece and the Greek Cypriot administration support a federation on Cyprus, Turkey and the TRNC insist on a two-state solution that reflects the realities on the island.