Four migrants drowned off the Greek island of Lesbos while trying to cross the sea from nearby Turkey, the coast guard said on Monday, adding that 18 others were rescued.
The coast guard did not give further details.
Greece is one of the European Union's main entry points for migrants and refugees fleeing poverty and conflict in the Middle East, Africa and Asia.
More than 15,600 people have arrived in Greece this year, according to data from the United Nations refugee agency UNHCR, nearly 12,000 of them by sea.
In June, hundreds of people drowned in international waters off Greece after the fishing trawler they were travelling on capsized en route to Italy from Libya.