Deep-water search operations to locate the wreckage and black boxes of the EgyptAir jet that plunged into the Mediterranean last week will start in the coming days, France's BEA air safety agency said Thursday.

"A deep-water search campaign will begin in the coming days with the arrival in the accident area of the French navy surveillance vessel 'La Place'," said the BEA, which is working alongside the Egyptian authorities to investigate the May 19 crash.

All 66 people on board the flight from Paris to Cairo were killed. Investigators are still searching for the Airbus A320's two black boxes on the seabed as they seek answers as to why the aircraft went down.

Two BEA investigators were on board the La Place ship as it set sail from Corsica on Thursday.

The vessel is equipped with three deep-water devices known as Detector 6000s that can detect the black boxes' signals, the French agency said.

The Egyptian authorities "will be piloting these underwater searches" with the BEA's help, it added.

Talks are still under way to add to the mission a second vessel equipped with a deep-sea exploration robot and the recovery capabilities required to work at an estimated depth of 3,000 metres (10,000 feet).

Time is running out because black boxes can only emit signals for about a month.

French Foreign Minister Jean-Marc Ayrault said Wednesday that Egypt was enlisting two French companies to help find the black boxes.

The two countries will share the costs.

Egyptian President Abdel Fattah al-Sisi has said it is too early to tell what caused the plane to plunge into the sea, and investigators have not ruled out terrorism as a possible cause.

Ayrault last week said "all theories are being examined and none is favoured".

The passengers were 30 Egyptians, 15 French citizens, two Iraqis, two Canadians, and citizens from Algeria, Belgium, Britain, Chad, Portugal, Saudi Arabia and Sudan. They included a boy and two babies.

Seven crew and three security personnel were also on board.

Earlier Thursday, hundreds of people gathered in Cairo for a candlelight vigil for the victims of the crash.

Clutching bouquets of flowers, candles and the Egyptian flag, around 500 mourners assembled at the Cairo Opera House and observed a minute's silence in honour of the victims.