The plane lost contact with airport control shortly after taking off from Beirut amid thunder and heavy rain and crashed into the water off Lebanon’s south coast.
Ethiopian Airlines flight 409 was carrying 83 passengers and seven crew.
Two Britons were among those on board the Boeing 737-800 to Addis Ababa the Foreign Office has said.
The cause of the crash has not been confirmed but Lebanon has been thrashed by stormy weather since yesterday, with crackling thunder, lightning and pouring rain.
Lebanese President Michel Sleiman said he did not think it was a terrorist attack.
“Sabotage is ruled out as of now,” he said.
“This is a painful, tragic event. We are sparing no efforts in trying to find survivors. Our heartfelt sympathy goes out to the families of those on board.
“The weather conditions are terrible, but rescue efforts are still under way.”
The Lebanese army and navy and the United Nations Interim Force in Lebanon are scouring the area alongside a 14-strong team sent by Ethiopian Airlines.
The chief executive of Ethiopian Airlines said he had no word of any survivors.
“I have contact with the Lebanese authorities, who have not yet confirmed any survivors,” Ethiopian Airlines CEO Girma Wake said.
He said the aircraft had been serviced on December 25 and passed inspection.
The Lebanese government has declared a day of mourning.
Prime Minister Saad al-Hariri visited the airport to meet distraught relatives waiting for news of survivors, some of whom were angry that the plane was allowed to take off in bad weather.
“They should have delayed the flight for an hour or two to protect the passengers,” a relative of one of the passengers told a local television station.
“There had been strong lightning bolts and we hear that lighting strikes at planes especially during take-offs.”
Fifty-four of those on board were Lebanese, 22 were Ethiopian, two were British and there were also Canadian, Russian, French, Iraqi and Syrian nationals.
Marla Pietton, wife of the French ambassador to Lebanon Denis Pietton, was on the plane, the French embassy said.
