Authorities launched a search operation Monday for missing migrants after a boat carrying them overturned in the Mediterranean Sea south of the island of Crete, officials said.

The operation was taking place about 20 nautical miles (37 kilometers) south-southwest of Gavdos, Greece’s southernmost island.

Authorities said 48 migrants had been rescued, while search efforts continued with a Greek Coast Guard vessel, a rescue craft and a vessel operated by the EU’s Frontex border agency.

Officials also said a vessel from the Libyan Coast Guard rescued 39 survivors.

Authorities did not immediately say how many people had been aboard the boat or how many remained missing.