A police official says the brakes of the bus heading to the capital city Nairobi are suspected to have failed.
At least 36 people were killed and 11 injured early Sunday morning in a head-on collision between a bus and a lorry on a road in central Kenya, police said.
"The death toll is now 36," said Rift Valley traffic police chief Zero Arome, explaining the initial toll of 30 had risen, "after six passengers succumbed to injuries in hospital."
The accident occurred at 3:00 am (0000 GMT) close to a notorious stretch on the Nakuru-Eldoret highway.
A bus travelling from Busia, in western Kenya, collided with a truck coming from Nakuru town.
In recent weeks road accidents have claimed the lives of hundreds of people, among them three Pentecostal bishops and a newly elected governor.
Police said the death toll for that stretch of road has now topped 100 this month alone.
Arome said the drivers of both vehicles were among the dead, as well as a three-year-old child, while the injured had been taken to a Nakuru hospital.
He added that the bus's brakes are suspected to have failed.
One survivor, speaking from his hospital bed, said he had been asleep at the back of the bus when the collision happened.
"All I heard was a loud bang and screams from all over," he said. "I was seated at the back and was helped out after some time because my legs were stuck. It is by the grace of God that I am alive. I saw many people dead and their bodies mutilated."
Official statistics show that around 3,000 people die annually in road accidents in Kenya, but the World Health Organization (WHO) estimates the figure could be as high as 12,000.
In December last year more than 40 people died when an out of control fuel tanker ploughed into vehicles and then exploded on another busy stretch of highway.
Deaths from road accidents commonly spike during the holiday period when people criss-cross the country visiting relatives.
While authorities have blamed careless road users, unroadworthy vehicles and speeding for the accidents, other observers say poor road construction and maintenance are to blame.