A Kenya Airways plane with 114 people on board has reportedly crashed in southern Cameroon.
The flight, which originated in Ivory Coast, was reported missing on Saturday after it failed to arrive in Kenya.
Cameroon state radio said the plane came down south of Douala, although Kenya Airways has only confirmed so far that it is missing.
People from at least 23 different nationalities were on board, including five Britons, the airline said.
Kenya's national carrier has a good safety record. However, 169 people died when one of its planes crashed in 2000.
The BBC's Karen Allen in Kenya's capital, Nairobi, says the Boeing 737-800 involved in Saturday's incident was just six months old and was part of a new fleet bought by the airline.
Our correspondent says it will raise questions of whether other aircraft will be taken out of service.
Flight KQ 507 originated in Abidjan, in Ivory Coast, and left Douala, in Cameroon, at 0005 local time (0105 GMT) on Saturday. It was due to arrive in Nairobi at 0615 (0315 GMT).
Kenya Airways said the last communication with the missing plane was received by the control tower in Douala, on Cameroon's coast, shortly after take-off.
Cameroon radio initially said the plane came down near Niete, south along the coast from Douala, although spotter helicopters were later searching for wreckage further inland - near the town of Lolodorf.
"The search location has now been centred around 100kms (62 miles) south-west of [Cameroon's capital] Yaounde," Kenyan Airways chief executive Titus Naikuni told a news conference.
He said an extensive search of the area by low-flying aircraft had found nothing, and a second search team was on its way to the site.
Mr Naikuni said the search and rescue operation was proving to be difficult because it was taking place in the heavily-wooded terrain.
Kenyan Transport Minister Chirau Ali Makwere - who is leading a team of Kenya Airways and government officials to Douala - said it was too early to determine what had happened to the plane.
"We need to get information from the technical experts as to whether it was occasioned by the weather or pilot error or mechanical fault," he was quoted by the Associated Press as saying.
NATIONALITIES OF MISSING
9 Kenya (crew)
7 South Africa
6 Ivory Coast
2 Central African Republic
2 Democratic Republic of Congo
2 Equatorial Guinea
1 Ghana; Sweden; Togo; Mali; Switzerland; Comoros; Egypt; Mauritius; Senegal; Congo; Tanzania; US; Burkina Faso
Source: Kenya Airways
A crisis management centre has been set up in Nairobi.
Five Britons were reportedly among the passengers, including Anthony Mitchell, a respected and well-known Associated Press journalist based in Nairobi.
There has been no confirmation from the Foreign Office.
There have been distressing scenes at Nairobi's Jomo Kenyatta International Airport where a number of worried friends and relatives have gathered.
"We can only hope for the best and pray... We're anxious and desperate," one man said.
The Kenya Airways website says the fleet is 23 strong. It is 26%-owned by Air France KLM's Dutch company KLM.
In January 2000 a Kenya Airways plane crashed into the sea after taking off from Abidjan airport in Ivory Coast killing 169 people. There were 10 survivors.