Twenty people were killed and seven were injured, two critically, when their bus collided at high speed with a broken-down truck in central Chad, the transport ministry said on Friday.

The accident occurred shortly before midnight Thursday on a desert road at Oum-Hadjer, 500 kilometres (300 miles) east of the capital N'Djamena, it said in a statement.

The coach was heading from N'Djamena to Ouaddai province with 31 people onboard.

Early investigations pointed to excessive speed and driver carelessness or fatigue, it said.

Eighteen people died on the spot and two at a hospital in Abeche, where the injured were also taken.

Deadly accidents are frequent in Chad, a vast and impoverished country where roads and vehicles are poorly maintained and driving standards are often lax.

In February last year, 33 people were killed and 54 injured when two coaches collided at night on the same road, not far from Oum-Hadjer.