Around 100 people were rescued, but it remained unclear how many were on board when the vessel went under, the authorities said, Reuters reported.
The shipwreck was the deadliest off Greece in 2023.
The coast guard said the boat was en route to Italy.
It was spotted in international waters late on Tuesday by an aircraft belonging to European Union border agency Frontex and two nearby vessels, around 80km south-west of the town of Pylos in southern Greece.
It said those on board had refused assistance offered by Greek authorities late on Tuesday.
A few hours later the boat capsized and sank, triggering a search and rescue operation.
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Survivors from a fishing boat that sank off southern Greece in one of Europe's worst migrant disasters say up to 100 children may have been on board.
ReplyDeleteAt least 78 people have already been confirmed dead in the disaster.
But scores more could still be missing at sea, with reports suggesting that up to 750 people were aboard the vessel.
The country's coastguard has been criticised for not intervening earlier but authorities say their offers of aid were refused.
Rescuers are still scouring the seas off Greece in a massive search operation, as hopes of finding more survivors dwindle.
BBC