Turkey has blocked a request from Cyprus to participate as an observer at the Conference on Disarmament in Geneva, amid growing tensions between the two countries.
Cyprus was one of a long line of countries to request the right to serve as observers in the first session of the year of the 65-member Geneva-based body.
All but one request were accepted by the necessary consensus.
But when the request from Cyprus was raised, Turkey's representative requested the floor.
He said Turkey has in the past refrained from blocking Cyprus's participation, and had simply voiced its displeasure in a letter at the end of each session.
However he said that has changed, stressing that "Turkey will not support this particular request this year."
The United States and European Union lamented the Turkish move.
"It is unfortunate that Turkey has decided to block Cyprus from participating in the work of the conference," said the US ambassador to the conference, Robert Wood, urging "Turkey to reconsider its objection."
Turkey's objection came amid growing tensions over Ankara expanding its claims over a large gas-rich area which Cyprus says includes its territorial waters.
Cyprus has aligned itself with Greece, Israel and Italy in planning an ambitious 2,000-kilometre eastern Mediterranean gas pipeline to Europe despite Turkey's hostility to the deal.
Cyprus is divided between the Greek Cypriot-run south - an EU member state - and the Turkish Cypriot north, where Ankara has stationed tens of thousands of troops since its 1974 invasion following an Athens-engineered coup.
Turkey insists the Turkish Republic of Northern Cyprus - recognised only by Turkey - has the right to explore in the island's waters.
Cyprus has also joined forces with Greece, Israel, Egypt and France in condemning a maritime and security accord that Turkey signed with Libya's UN-recognised government.
Turkey has been flexing its muscles across the Mediterranean from Libya to Syria, where Ankara has taken on diplomatic and military roles.
Analysts have warned that Turkey sees Cyprus as the weakest link in the regional alliance buffering Ankara's ambitions.
The Conference on Disarmament is a multilateral disarmament forum that meets in three sessions a year in Geneva. It negotiates arms control and disarmament accords and focuses on the cessation of the nuclear arms race.