A fireball from an oil lorry engulfed vehicles on one of Kenya's main highways, killing at least 42 people late on Saturday, a rescue worker at the scene said.

The tanker lorry rammed into vehicles north of the central town of Naivasha then exploded, an official with the National Disaster Management Authority said.

"More bodies are still trapped inside the burned vehicles, " Red Cross volunteer Moha Maris told Reuters.

The government gave a lower death toll. "The information that we have is that 13 vehicles were involved and so far we have retrieved 33 bodies," Irungu Nyakera, Principal Secretary at the Ministry of Transport and Infrastructure, told a news conference.

An official in Naivasha said 50 people were being treated in several hospitals for burns, eight of them in a serious condition.

In a statement issued by Kenya's State House, President Uhuru Kenyatta said "the tanker responsible for the deaths should not have been on that particular road at that hour" and called for a full investigation into a "tragic breach" of the rules restricting the movement of large trucks on certain roads and at certain times.

Police and witnesses said the truck with Ugandan registration plates was travelling fast when it hit a speed bump and then lost control, reports AFP.