BEIRUT — Search crews used sonar Thursday to scan the sea floor for a crashed Ethiopian Airlines jet after a U.S. Navy vessel picked up a beacon from the plane's black box almost a mile below the surface.

The Boeing 737 crashed into the Mediterranean Sea early Monday just minutes after takeoff from Beirut in a fierce thunderstorm. All 90 people on board were feared dead.

A U.S. Navy ship, the USS Ramage, detected signals from the black box flight recorders at 1,300 metres under the sea, the Lebanese army said Thursday. It could take days to find and retrieve the black box, which is key to determining the cause of the crash.

Aladar Nesser of U.S.-based Odyssey Marine Explorations, which sent a ship to help in the search, said cables equipped with sonar stretch nearly 3 kilometres behind the ship and down to within 50 metres of the sea floor.

"That provides a mapping of what's down on the bottom of the sea," Nesser said. "That imagery is good enough to be able to distinguish the wreckage of the airplane."

The location is far too deep for human divers and remotely operated devices would have to be used in a retrieval operation, he said.

Walid Noshie, a prominent Lebanese diver who also is helping the search, said the priority is to find the people on the plane, and then the black box.

Rescue teams have recovered some bodies and pieces of the plane, but hope for finding any survivors has faded. There are conflicting numbers of how many bodies have been found, although at least a dozen have been pulled out of the chilly waters.

The flight recorders are critical to determining what caused the crash. Lebanon's transportation minister has said the plane made a "fast and strange turn" after takeoff, and weather experts said lightning struck in the flight path around the time of the crash.

The Lebanese army and witnesses reported the plane was in flames as it went down.

Health officials have handed over to families the remains of several victims who were identified using DNA, Health Minister Mohammed Jawad Khalife said.

Many in Lebanon have already begun grieving for the crash victims.

Weeping relatives gathered in Beirut to mourn 2-year-old Julia al-Hajj, whose coffin was covered in flowers and a picture of the girl wearing a pink outfit with a tiny black bow.

Schoolchildren also gathered on the beach in Khalde, south of Beirut, on Thursday and threw yellow and orange flowers into the sea in a ceremony of mourning over the crash.

Lebanese officials also plan to send a team to Ethiopia to take DNA samples of victims' families there, Lebanon's state-run news agency said Thursday.