Forty-six people, including a dozen minors, were killed after a bus caught fire south of the Bulgarian capital early Tuesday, officials said, in the country's deadliest road accident in years.

A cause has yet to be determined but officials believe a fire broke out on board the tourist bus which was travelling from Turkey to North Macedonia. It then crashed into guardrails.  

There were no other vehicles involved in the accident, which occurred around 2am (0000 GMT) on a highway about 40 kilometres (26 miles) from Sofia, near the village of Bosnek, and seven people survived. 

"Unfortunately, there's many victims, 46, including 12 minors," North Macedonian Health Minister Venko Filipce told public BNT television after visiting survivors in a Sofia hospital.

The seven survivors -- two women, including one aged 16, and five men -- are all from the same family, he said. Among the dead were victims of several nationalities, including a Belgian.

Bulgaria's interim Prime Minister Stefan Yanev said a probe into the accident had been launched, dismissing the suggestion that road conditions were to blame.

Images showed the carcass of the totally burnt-out bus after it broke through the guardrails between the two sides of the highway.

Officials work on the site where 45 people were killed, including minors. Photo: AFP

'Terrifying scene'

According to bTV, the bus was travelling from Turkey's main city of Istanbul to Skopje in North Macedonia.

Local media in North Macedonia report that the bus was registered to the "Besa trans" tourist agency, which organises touristic and shopping tours to Istanbul.

North Macedonian Prime Minister Zoran Zaev told state news agency MIA that he had spoken to one survivor.

"He explained that they were sleeping in the bus when an explosion was heard. They succeeded to break one of the windows and save a few people. Unfortunately, the rest did not succeed," he said.

"It is a great tragedy... 12 (victims) are under 18 years old, and the rest are young people about 20 to 30 years old," he added.

Bulgarian national police chief Stanimir Stanev while the two drivers of the bus were Macedonian, the passengers were Albanian, according to initial information.

Stanev said the bus driver died "immediately so there was no one able to open the doors".

Survivors "jumped out of the windows", said Maya Arguirova, head of the treatment centre for severe burns where they were transported.

All survivors were in stable condition.

Bulgaria's Interior Minister Boyko Rashkov was among those who rushed to the site of the crash early Tuesday.

"It's a terrifying scene. I haven't seen anything like that before," Rashkov told journalists at the site. 

"Nobody can say for certain how many are there and who they were. The bodies are badly burned and have to be identified one by one," he added.

Bulgaria has a history of deadly bus accidents. Twenty Bulgarians died in 2018 when their bus skidded on a wet road and overturned.

A total of 628 people died in road accidents in 2019 and 463 in 2020 in the country of 6.9 million people, according to official data. The accidents were often attributed to poor road conditions, outdated cars and speeding.

Tuesday's accident occurred on a section of highway with steep gradients and without clear demarcation lines, where many accidents have taken place, according to Diana Roussinova of a road safety NGO, which has complained to authorities about the stretch in the past.