A cargo ship believed to be carrying hundreds of migrants has turned towards Italy, brushing off Greek authorities responding to a distress call from a passenger.

A passenger on board the Moldovan-flagged Blue Sky M called emergency services in Greece earlier in the day when it was sailing off the coast of the island of Corfu in the Ionian Sea.

Greek state television earlier reported a passenger ship issued a distress signal because of armed people on board.

Greek authorities scrambled a navy frigate and a helicopter to check the ship and the nature of the emergency. The ship was believed to be carrying between 400 and 600 migrants.

The passenger's phone call said the ship was about 10 miles off Corfu with a large number of migrants, said Admiral Nikolaos Patrinos, head of the coastguard's security operations. The passenger said: "We are without water, food and blankets."

A coastguard official said the Greek frigate had contacted the cargo ship's captain, who said the ship was not in any danger and was not requesting assistance. The ship headed towards Italy.

Tens of thousands of migrants and refugees, most fleeing conflict or poverty in the Middle East, Africa and Asia, attempt to reach the European Union each year, with many making the perilous crossing by sea to Italy or Greece.

On Sunday, Greek passenger ferry Norman Atlantic caught fire in the same area, leading to a massive two-day rescue operation by Italian and Greek coastguard and military officials.

The Greek frigate had been in the area assisting in the Norman Atlantic rescue operation.