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 2009, more than 100 people burned to death near the central town of Molo after a lorry carrying petrol caught fire.