Sixteen people, including at least five children, drowned when the small boat they were travelling on capsized in the Aegean Sea, Greek coast guard officials said on Saturday.
"At least four more were unaccounted (for)," a coastguard official told Reuters. Three other migrants were rescued by Greece's Agathonisi island near the Turkish coast.
It is the highest number of migrant casualties in months, after hundreds have lost their lives in recent years crossing the Mediterranean seeking to escape war, poverty or both.
"According to the migrants that have been rescued, there were 21 people on the boat," the official added.
Coastguard vessels assisted by two helicopters were searching for more survivors.
Although migrant flows to mainland Greece, which shares a long sea border with Turkey, have slowed, many continue to reach the Greek islands.