Migrants are seen on a capsizing boat before a rescue operation by Italian navy ships "Bettica" and "Bergamini" (unseen) off the coast of Libya in this handout picture released by the Italian Marina Militare on May 25, 2016. Marina Militare/Handout via REUTERS

At least 700 migrants were believed to be on board a boat that capsized off the Greek island of Crete Friday, with hundreds still missing, the International Organization for Migration said.

“Over 250 people have been rescued from a shipwreck off Crete which is believed to have left Africa with at least 700 migrants on board,” the organisation said in a statement.

Any confirmed deaths “would be the first fatalities in Greek waters since April,” the IOM said, after a controversial deal in March between the EU and Turkey led to a sharp drop in the flow of migrants to Greece.

In the most recent deadly incident in the Aegean, four women and a child drowned in early April off the island of Samos.

More than 205,000 migrants and refugees have crossed the Mediterranean to Europe so far this year.

Three quarters of them, mainly refugees fleeing conflicts in Syria and Afghanistan, arrived in Greece before the end of March, with 376 dying trying to make the perilous journey, according to IOM figures.

Along all Mediterranean routes, more than 2,500 people have died trying to reach to Europe since the beginning of the year — the vast majority of them on crossings between Libya and Italy.

Friday’s tragedy marked the second migrant boat found in that area of the southern Aegean Sea since last week, indicating that people smugglers may be forging a possible new route.

The IOM said its observations supported that theory, reporting a “surge of new arrivals to Greece further south, on sea lanes connecting North Africa to the island of Crete.”