The helicopter was attached to the Canadian frigate Fredericton, from which it had taken off for a patrol.

"There is a developing situation regarding one of our CH-148 Cyclone helicopters, deployed onboard HMCS FREDERICTON, which is currently contributing to Op REASSURANCE," the Canadian armed forces said in a statement.

"Contact was lost with the aircraft as it was participating in Allied exercises off the coast of Greece. Search and rescue efforts are currently underway," it added.

The statement did not give details on how many personnel were on board, although a Greek military source earlier said it was believed the aircraft had been carrying six people.

Initial reports said the helicopter had been over international waters 50 nautical miles off the Greek island of Kefalonia, the Greek air force said.

Italian, Greek and Turkish frigates also took part in the patrol that had moved from Greek to Italian waters on Wednesday, with the Greek frigate remaining in Greek waters.

Greek naval and air forces were ready to assist in a rescue operation "if Italy requests it", a military source told AFP.

A spokesperson for Supreme Headquarters Allied Powers Europe (SHAPE), which plans and carries out all NATO operations, added: "I confirm there has been an incident involving a helicopter from a ship under NATO command.

"There is currently a search and rescue operation being conducted and national notifications taking place."