A Kenya Airways airliner with 115 people on board crashed on Saturday after taking off from Douala airport in Cameroon on a flight to Nairobi, an aviation official said.
"The plane has crashed," the official with the Kenyan Civil Aviation Authority, who asked not to be named.
A top government official said, "We have located the plane ... and we cannot talk about it."
The plane, a Boeing 737-800 carrying 106 passengers, eight crew members and a flight engineer, was due to land in Nairobi on Saturday at 6.15 am (0315 GMT).
Air traffic controllers picked up a distress signal from the missing Kenya Airways airliner just after it took off from Douala airport in Cameroon, an aviation source said.
The signal was picked up by satellite at 2.00 am (0100 GMT), a source close to the Agency for the Safety of Aerial Navigation in Africa said.
Two Cameroonian army helicopters on Saturday began searching a wide area south of the line between Douala and the capital Yaounde, 250 kilometres (180 miles) to the east, to try to locate the aircraft.
Cameroonian authorities have set up two emergency centres, one in Yaounde and the other in Douala, to coordinate search operations.
Flight KQ 507 was flying from Abdijan in Ivory Coast to Nairobi via Douala, in Cameroon.
The chief executive officer for Kenya Airways, Titus Naikuni, earlier told a news conference in Nairobi that the aircraft had failed to land.
"The last message from the aircraft was received at the control tower (in Doula) immediately after take off. After, the tower lost contact with the aircraft. So far, no one has been able to establish contact," Naikuni said.
Most of the passengers were connecting through Nairobi to other destinations outside Kenya. A crisis centre has been set up at the company headquarters at Nairobi airport.
Some 10-15 people were waiting at the arrivals section of Nairobi airport early on Saturday, guarded by several police officers.
"We have been told to sit and wait," said one relative of a passenger on the plane. "A lot of people are crying, people are asking what is happening."
Kenyan government spokesman Alfred Mutua said the government was on alert. "We are working on modalities to assist in any rescue operation," he said.
In January 2000 a Kenya Airways Airbus crashed into the sea after taking off from Abidjan airport in Ivory Coast killing 169 passengers and crew.
Kenya Airways was to hold a news conference at 12.00 pm.