An Ethiopian Airlines plane with about 90 people on board crashed into the sea shortly after take-off from Beirut, Lebanese aviation and security officials said.
The plane took off at around 2am local time and was heading to Addis Ababa, the Ethiopian capital.
About 50 of the passengers were Lebanese, the officials said.
Police ruled out terrorism as the cause of the crash and said it was probably weather-related.
It has been raining heavily with lightning in Beirut since last night.
Lebanese naval ships were scrambled to find the location of the Boeing 737-800 crash.