At least 50 people have been killed after a bus travelling from the Kenyan capital Nairobi to Kisumu, in the west, veered off the road, police say.

Many reportedly died when the vehicle overturned. Police fear the death toll may rise, with the possibility of passengers trapped in the wreckage.

"The roof of the bus was ripped off," a local official said.

Police said the driver had lost control and plunged down a slope at a notorious accident black spot.

"It is unfortunate that we have lost 51 people," Kenya's police chief Joseph Boinnet told Capital FM radio.

The bus was carrying 52 passengers at the time of the crash in the early hours of Wednesday morning, police said.

Children are believed to be among the dead.

An eyewitness described hearing "a long hooting sound, then a screech that was followed by a bang and loud screams," Kenyan newspaper Daily Nation reported. 

The newspaper added that a number of people had been taken to a local hospital for treatment.

Highway roads in Kenya can be notoriously dangerous and the World Health Organization (WHO) has ranked Kenya as one of the world's worst countries for road safety.

Last December, some 36 people died in a head-on collision between a lorry and a Nairobi-bound bus on a road near Migaa in Kenya.

In 2016, several vehicles were engulfed in flames in a crash involving an oil tanker in Naivasha, Kenya, which killed dozens of people.