The Kenya Red Cross tweeted that the tanker, heading to Uganda, had appeared to lose control before crashing into other vehicles in Naivasha, north-west of the capital Nairobi.
Eyewitness Moses Nandalwe told the BBC a huge fireball had quickly engulfed other vehicles on the busy road.
He feared that many people who had gathered at the scene of the crash were also caught in the inferno.
Mwachi Pius Mwachi, deputy director of Kenya’s National Disaster Management Unit, said the tanker had been travelling downhill when it went out of control.
Correspondents say the accident has happened at a time when doctors are on strike, severely limiting services in government hospitals.
The inferno consumed at least 11 vehicles which have been burnt beyond recognition and at least 30 people have been confirmed dead.
The accident involved a petrol tanker, a passenger bus and several vehicles. The tanker reportedly rammed into 5 vehicles before exploding into flames.
Terrible accident being reported near Naivasha, involving, a petrol tanker, a passenger bus and several vehicles. @ntvkenya
— Jamila Mohamed (@JamilaMohamed) December 10, 2016
Witnesses at the scene said the driver of the tanker lost control and hit the other vehicle which created chain of knocks.
The accident on the busy highway, some 80 kilometres west of Nairobi created a horrible scene.
NTSA boss Francis Meja confirmed that 12 vehicles have been burnt, adding that one is a PSV and the rest are private.
The matatu was carrying 14 passengers, who have all perished.
The actual death toll could be higher given the number of vehicles involved.
— SokoAnalyst (@SokoAnalyst) December 10, 2016
The tanker, which was coming from Nairobi, was negotiating the hilly part of the road, before knocking a matatu and exploded into flames.
Earlier, a Kenya Red Cross official said rescuers had taken 30 bodies to Naivasha Sub-County Hospital Mortuary.
The accident happened on the fifth day of a national strike by doctors and nurses, something that could worsen the conditions of the casualties.
Karai is about five kilometres from Naivasha Sub-County Hospital, one of the facilities affected by the industrial action.