Police sources said an investigation had been opened into Monday's blast to determine if it had been caused deliberately.

Yousouf Saleh Abbas, Chad's prime minister, went to the scene and later visited the injured in hospital.

The rebels launched a surprise attack in February after dashing more than 1,000km across the country aboard about 300 pickup trucks and came close to overthrowing the regime of Idriss Deby Itno, the country's president, before being repulsed by government forces.