A gasoline truck exploded after smashing into a bus in southeast Iran late on Thursday, killing 90 people and injuring more than 100, officials said yesterday.

"The disaster is so grave we cannot identify faces and cannot differentiate between corpses," Haidar Ali Nourai, governor of the southeastern city of Zahedan, told state television.

He said the accident happened when the truck lost control and ploughed into a bus waiting at Nosratabad police checkpoint. The fireball then enveloped five other buses.

Television showed bodies charred beyond recognition. A gold watch was seen stuck to an arm scorched to the bone.

Red Crescent spokesman Mehran Nourbakhsh said 90 people had died in the blast and 114 had been injured. The aid group has sent 40 workers to the scene.

Zahedan parliamentarian Hossein Ali Shahriari said the checkpoint was badly positioned on a sharp bend.

"Inspections at this checkpoint use stone-age methods," he told the ISNA student new agency. "We always see lots of buses and cars caught in queues at this road block."

Iran has one of the highest road accident rates in the world, averaging five deaths every two hours.

Nosratabad lies on the road from Zahedan to Bam, which was hit by an earthquake last year that killed more than 20,000 people.

Zahedan is about 30 kilometres from the Pakistan border as the crow flies.

The road is dotted with checkpoints because it is the main route for drugs smugglers taking opiates from Afghanistan and Pakistan to the West.

Around 300 people were killed and 450 injured in February when a train laden with gasoline and fertiliser exploded in the northeast of the country after derailing.

The force of the blast near Nishapur razed village homes to the ground, crushing people under crumbled mud brick walls.