A vessel that called for help off Greece is a Moldovan-flagged cargo ship believed to be carrying migrants, Greece's shipping ministry said on Tuesday.

Greek state television earlier reported a passenger ship issued a distress signal because of armed people on board. Greece is sending a navy frigate and a helicopter to the ship.

The ship, the Moldovan-flagged Blue Sky M, was sailing in poor weather near the tiny island of Othonoi, northwest of Corfu. Authorities said the distress call had come from one of the passengers on board the ship.

The ship is believed to be carrying about 600 migrants.

The incident comes two days after a Greek passenger ferry with more than 400 people on board caught fire in the same area, leading to a huge rescue operation by Italian and Greek coast guard and military officials. At least 10 people died in that incident, while more than 400 were rescued.