Twelve migrants, believed to be Syrians, were found dead in western Greece on Friday after a plastic boat still tied to rocks capsized in calm weather, authorities said.

Another 15 people survived the incident that occurred near the island of Lefkada and prompted Greece's left-wing main opposition party to accuse the European Union of turning the Mediterranean into a "sea of death."

Coast guard divers recovered the bodies of four children and eight adults after survivors alerted authorities, the Merchant Marine Ministry said. The migrants were presumed to have been headed to nearby Italy from the western Greek mainland.