At least 33 people were killed, most of them children, when a bus burst into flames after colliding with a gas tanker in Morocco on Friday.

"We have many completely carbonised bodies. Authorities have been in contact with the bus company to identify the victims," the transportation ministry said.

The bus was heading from the coastal city of Benslimane to Laayoune in Western Sahara when the crash happened near the southern city of Tan-Tan.

Most of the victims were children returning from a school athletic competition in Benslimane, the Tan-Tan bureau of the Moroccan Association of Human Rights said in a statement.

The bus collided with the gas tanker at Shbeka town, 60km south of Tan-Tan, it said.

Ten people were injured and five had left hospital after treatment, it added.