The accident happened around 2 am on Tuesday. Seven people were taken to hospitals for treatment.

The cause of the crash was not immediately confirmed, but it appeared that the bus hit a highway guard rail, crashed and caught fire.

The bus was one of four travelling together. Officials said an investigation will be launched.

Photos taken shortly after the crash showed the bus engulfed in flames with plumes of thick, black smoke rising from the scene.

Interior Minister Boyko Rashkov told reporters at the crash site that he had "never in my life seen something more horrifying."

"The picture is horrifying, the people who were on the bus are turned to charcoal," Rashkov said. "It is impossible to say how many they were. There were four buses that travelled together, and it is possible that passengers changed buses during the stops."

Bulgarian Caretaker Prime Minister Stefan Yanev, who also visited the site of the crash, told reporters it was "a huge tragedy."

"I take this opportunity to send my condolences to the relatives of the victims," Yanev said. "Let's hope we learn lessons from this tragic incident and we can prevent such incidents in the future."

Bulgarian news agency Novinite said representatives from North Macedonia's embassy visited a hospital where some of the victims were taken.

North Macedonian Prime Minister Zoran Zaev told Bulgarian television channel bTV that he had spoken to one of the bus survivors.

"One of the passengers told me that he was asleep and woke up from an explosion," Zaev told bTV, adding that the authorities will gather information that is "important for the families of the dead and the survivors."

In 2019, Bulgaria, an EU nation of 7 million, had the second-highest road fatality rate in the 27-nation bloc, with 89 people killed per million population, according to European Commission data.

Australian Associated Press