The jet bound for the Kenyan capital, Nairobi, went down near the town of Lolodorf, about 155 miles south of the coastal city of Douala, where it had taken off after midnight, said Alex Bayeck, a regional communications officer.
There was no word on survivors, said Bayeck, speaking by telephone en route to the crash site. He said search planes were flying over the forested area where the airliner gave off a distress signal, but no wreckage had been spotted.
He said residents in the area, which has few roads and is dotted by small villages, reported hearing a "large boom" during the night. "Searchers have gone out looking in this area," he said.
Kenya Airways CEO Titus Naikuni said the plane, which was almost new, took off an hour late because of rain. He said the distress call was issued automatically "from a machine, not a pilot."
Douala airport officials confirmed heavy rain at the time of takeoff but said that was unlikely to have caused the accident.
"There was a thunderstorm, but there were other planes that left after (the Kenya Airways flight to Nairobi) that had no problems," said Thomas Sobatam, head of weather observation at the airport.
Kenya's transport minister, Ali Chirau Makwere, said it was too early to determine what caused the crash.
"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 said in Nairobi. "We really don't know. It's too early to make any conclusions."
The Boeing 737-800 was carrying 114 people, including 105 passengers, from at least 23 countries, Kenyan airline officials said. A Nairobi-based Associated Press correspondent, Anthony Mitchell, was believed to be among the passengers. Mitchell had been on assignment in the region for the past week.
"Anthony ... had contacted his family before boarding the flight to let them know he was headed home. We hope for the best," AP Executive Editor Kathleen Carroll said.
Relatives waiting at Nairobi's airport began wailing as news reports of the crash filtered in. Dozens of family members collapsed in the airport terminal.
One person at the airport said families had not been given any information. "I cannot talk now because there is no news," he said, declining to give his name.
Janet Mwema went to a crisis center Kenya Airways set up at a Nairobi hotel because she believed her daughter, Vicky, a cabin crew member, might have been on the flight. She said the counselors took her name and phone number but couldn't confirm whether her daughter was on the flight.
The flight departed Douala at 12:05 a.m. and was to have arrived in Nairobi at 6:15 a.m. The flight originated in Ivory Coast but stopped in Cameroon to pick up more passengers, the airline said.
"The last message was received in Douala after takeoff and thereafter the tower was unable to contact the plane," Kenya Airways' Naikuni said earlier Saturday.
The Douala-Nairobi flight runs several times a week, and is commonly used as an intermediary flight to Europe and the Middle East.
Kenya Airways considered one of the safest airlines in Africa said most passengers were planning to transfer to ongoing flights in Nairobi.
Naikuni said the plane was only six months old.
U.S. aviation officials are standing by, expecting to head to Cameroon and help with the investigation if asked.
"Generally, other governments don't have the same accident investigation expertise that we have," said Laura Brown, a spokeswoman for the Federal Aviation Administration.
In 2004, the United States helped investigate the crash of a Flash Airlines Boeing 737 that killed all 148 on board minutes after taking off from the Red Sea resort of Sharm el-Sheik, Egypt.
Boeing spokesman Jim Proulx said there have not been any safety concerns with the Chicago-based company's fleet of 737-800s. About 2,000 737-800s are in use around the world.
"We express our profound concern for the passengers and crew on board on the Kenya Airways flight that went missing," Proulx said Saturday. "We stand ready to assist the authorities if they ask us to do so."
The last crash of an international Kenya Airways flight was on Jan. 30, 2000, when Flight 431 was taking off from Abidjan, Ivory Coast, on its way to Nairobi. Investigators blamed a faulty alarm and pilot error for that crash, which killed 169 people.