ATHENS — The bodies of 12 people were recovered from the Ionian Sea on Saturday after a migrant smuggling boat sank near the island of Paxos in western Greece, the Greek Coast Guard said.

The authorities were alerted that the vessel was in trouble shortly after 9 a.m., when one of the migrants on board phoned the emergency services. No details were immediately available about the identities of the victims.

The cause of the sinking was also unclear, according to the Coast Guard spokesman, Nikolaos Lagadianos. Weather conditions had been good, he said. But the boat may have been overloaded, he added, noting that about 50 people may have been on board.

A large search-and-rescue operation was underway, the Coast Guard said, with six Coast Guard vessels scouring the area alongside two Air Force helicopters, a Navy helicopter and four fishing boats. The Coast Guard said that 21 people had so far been rescued.