At least 45 people, including 12 children, have been killed after a bus carrying mainly tourists from northern Macedonia was engulfed in flames on a highway in Bulgaria, hours before dawn on Tuesday.

The seven people who were thrown from the burnt bus were taken to Sofia hospital and are in stable condition, hospital staff said. They suffered burns and one broke his leg.

Interim Interior Minister Boyko Rashkov said the bodies were "piled up inside and burned." "The picture is scary, scary. I have never seen anything like it before," he told reporters in the country.

The cause of the accident is unclear, but the bus is thought to have hit a highway obstacle either before or after it was engulfed in flames, Bulgarian officials said. "The driver's human error or a technical malfunction are the two initial versions of the accident," said Bulgarian Investigative Service Chief Borislav Sarafov.

The accident happened on the Struma highway about 45km west of Sofia around 2am. The group of tourists was returning to Skopje from a weekend trip to Istanbul.

The Prime Minister of Northern Macedonia, Zoran Zaev, said that the passengers were from different communities in Northern Macedonia. Foreign Minister Olta Xhacka said the passengers were from the ethnic Albanian community of northern Macedonia.