At least 45 people, including 12 children, have died after a bus caught fire on a motorway in Bulgaria overnight.

A senior interior ministry official said the incident happened around 45km from the capital Sofia at around 2am local time.

Nikolai Nikolov said it was not clear whether the bus “caught fire and crashed, or crashed and then caught fire”.

He confirmed that children are among the dead and seven people who jumped out of the burning bus had been taken to hospital for treatment.

Most of the victims were tourists from North Macedonia, an official from the country's embassy told Bulgarian television channel BTV.

The country’s Interior Minister Boyko Rashkov said the images from the crash were terrifying – with ‘people clustered inside burnt to ash.”

North Macedonian Prime Minister Zoran Zaev said he had spoken to one of the survivors who told him passengers were sleeping when they were woken by the sound of an explosion.

“I am terrified,” he said. “This is such a huge tragedy.”

The cause of the incident is still unclear; however, Bulgarian investigators said the bus appeared to have struck a road barrier either before or after it caught fire.