Two British passport-holders were on board a jet which crashed into the Mediterranean, the Foreign Office confirmed.
The Ethiopian Airlines plane crashed into the sea off Lebanon in stormy weather just minutes after leaving the ground, with witnesses saying they saw a "ball of fire".
There has been no news of survivors among the 90 passengers and crew. At least 12 bodies were recovered as rescuers searched the stormy waters.
Lebanese President Michel Suleiman said terrorism was not suspected. "Sabotage is ruled out as of now," he said.
The Boeing 737-800 took off around 2.30am local time for the Ethiopian capital, Addis Ababa, said Ghazi Aridi, the public works and transportation minister. He said the plane went down about 2 miles off the Lebanese coast.
The Lebanese army said in a statement the plane was "on fire shortly after take-off".
Ethiopian Airlines' chief executive Girma Wake told journalists in Addis Ababa that he had no information on the fate of those on board or about the cause of the crash. He said the aircraft, leased from CIT Aerospace, had been serviced on December 25 and passed inspection.
Lebanese Prime Minister Saad Hariri announced a day of mourning and closed schools and government offices. Relatives of the passengers began arriving at the airport in Beirut, many of them crying and hugging, and were led to a VIP area.
The plane was carrying 90 people, including 83 passengers and seven crew. Mr Aridi, the transportation minister, identified the passengers as 54 Lebanese, 22 Ethiopians, one Iraqi, one Syrian, one Canadian of Lebanese origin, one Russian of Lebanese origin, a French woman and two Britons of Lebanese origin.
The wife of Denis Pietton, the French ambassador to Lebanon, was on the plane, according to the French embassy.