At least 78 migrants are known to have drowned when their boat capsized off the southern coast of Greece, with many more possibly missing.
Officials have been quoted as saying the vessel may have been carrying up to 400 people.
More than 100 have been rescued, but the operation has been hampered by strong winds.
The Greek coast guard said the boat had been spotted by a surveillance plane from the EU’s Frontex agency on Tuesday afternoon, but that the passengers refused any help.
Authorities said the vessel appears to have been on its way from Libya to Italy.