They had fled across the border to Bangladesh amid heavy fighting between the Myanmar military and rebel insurgents in Rakhine
Police found a woman among those killed and seven other passengers injured with burns, said Chakaria police's Inspector Kamrul Azam.
The accident occurred around a half-hour past Thursday midnight on the Chittagong-Cox's Bazar highway, he added.
All the victims were from the Cox's Bazar-bound microbus which caught fire after the explosion.
Those killed could not be identified immediately, but the microbus driver was among them, police said.
The bodies have been kept at the Chakaria Upazila Health Complex.
Police seized both the vehicles.