Witnesses say the impact trapped the drivers and a woman inside the wreckage. The fire spread quickly and left them with severe burns. Among the dead are two drivers, one from each truck, and a woman. Reports say the woman was pregnant.
Footage from the scene shows heavy traffic stretching along the road. Lorries and other vehicles have been forced onto the shoulders to get past the wreckage and reach the other side.
Passersby tried to put out the flames with whatever they could find. In the videos, no emergency responders are visible.
It is still unclear whether firefighters have brought the blaze under control. Authorities have not yet established what caused the crash.
The accident adds to the long list of deadly collisions on Kenya's major highways. Drivers using Mombasa Road should expect delays while the scene is cleared and investigators complete their work. Motorists should also take extra care and follow any diversions set by traffic officers.
