At least 15 migrants, including five children, have died when their boat capsized as they tried to cross from Turkey, the Greek coastguard has said.
About 20 are believed to have been on board, and several are unaccounted for.
Coastguard vessels and helicopters are searching for more survivors off the island of Agathonisi, close to the Turkish coast.
Saturday’s incident is thought to be the highest death toll of migrants trying to reach Greece in months.
Last year, the European Union saw the lowest number of detected illegal border crossings since the migrant crisis began four years ago, European border agency Frontex said last month, AFP reports.
The EU reached deal with Turkey in 2016, which saw a decrease in the number of people crossing to Greece.
Under the deal, anyone who arrives on Greek islands must be returned to Turkey unless they qualify for asylum.
So far this year, there have been close to 4,000 migrants arriving by sea in the Greek islands, according to figures from the UN refugee agency, the UNHCR.