"We believe that 22 people were killed in the accident," said Aminuddin Abdul Rahim, spokesman for the Fire and Rescue Department, who was at the accident site in central Malaysia.
"More than 15 others were injured and are being treated by rescue officials," he told AFP. "Search and rescue operations are still ongoing and we have yet to have a final figure for those dead and injured."
"All those killed in the accident are believed to be Thai nationals, this was a tourist bus filled with Thai tourists."
Zakaria Yusof, acting police chief in Perak state, said that "more than 20 people are believed to be dead".
"The bus was filled with Thai tourists," he told AFP, but was not able to confirm whether all the dead were Thai nationals. "We will know more when we have finished our operations."
The Star daily said the bus hit a divider and overturned as it travelled away from the Cameron Highlands, one of Malaysia's top tourist destinations.
Some of the approaches to the highlands are narrow, windy and treacherous, but Monday's accident occurred when the bus had reached level ground and was on a highway.
Bus crashes are relatively common in Malaysia, often involving interstate buses, and accompanied by revelations that drivers were unlicensed, speeding or under the influence of drugs to keep themselves awake.
Last week, four prison officers were killed when a bus lost control and plunged into a ravine after travelling from a sports tournament.
In October a collision involving two buses, a van and three cars claimed 13 lives on the nation's main North-South highway.
And a year ago, 10 people were killed when a double-decker bus struck a road divider in northern Ipoh state.