French President Francois Hollande confirmed on Thursday that the EgyptAir flight that disappeared from radar screens over the Mediterranean had "crashed".
"We must ensure that we know everything on the causes of what happened. No hypothesis is ruled out or favoured," he said in a televised address.
"Whether it was an accident or another hypothesis that everyone has on their mind -- a terrorist hypothesis... at this stage we must focus on our solidarity with the families and the search for the causes of the catastrophe," Hollande added.
The Paris prosecutor's office said its accident department had opened an investigation into the crash.
The EgyptAir flight from Paris to Cairo crashed into the Mediterranean between the Greek islands and the Egyptian coast early Thursday with 66 people on board.
"We feel solidarity and compassion. It's not the first such catastrophe, and we know what it means for families and loved ones," Hollande said.
French Foreign minister Jean-Marc Ayrault (centre) leaves a hotel where relatives of passengers of missing MS804 Egyptair flight are gathered next to Roissy-Charles De Gaulle airport in Paris on Thursday.