Bulgaria, North Macedonia and Kosovo are mourning the 46 people were killed in one of Europe's worst road accidents.

A tourist bus crashed into highway rails and caught fire near Sofia in Bulgaria. It was returning from Istanbul in Turkey and was on its way to Skopje in North Macedonia. 

Only seven people survived, who managed to escape by breaking a window and jumping out of it, according to initial accounts. 

A list of passengers on the trip that was released by Skopje media suggested most of the victims, who included 12 children, were from North Macedonia's ethnic Albanian minority. 

Investigators have been sifting through the ashes of the burned-out vehicle to determine whether a technical defect or human error caused the crash. It still remains unclear exactly how many people were on board. 

The survivors were hospitalized in Sofia and in a stable condition, despite their injuries and bad burns, according to Maya Argirova, head of the Pirogov burns clinic.

The bus was registered with the Besa Trans tourist agency, which organizes tours to Istanbul, and was the last of four from the company to return from the trip. North Macedonia's transport minister told reporters on Tuesday the vehicle was not registered for international transport. 

Around 250 pupils from Skopje's Ismail Qemali school laid flowers at a monument to medieval Albanian hero Skanderbeg in tribute to five classmates, all from the Jahi family, who were killed in the crash with their mother. 

Tuesday's accident occurred on a notorious section of highway with steep gradients and unclear demarcation lines. 

The government in North Macedonia declared three days of mourning and ordered flags fly at half-mast. Bulgaria and majority ethnic Albanian Kosovo also declared Wednesday a day of mourning for the victims. 

In Bulgaria's capital Sofia people placed flowers and toys in front of the North Macedonian embassy to express their sympathy and condolences.

Passenger Lulzim Sulejmani recounted to local media what he remembers happening at the time, saying the bus hit something on the right, the wheels burst and the fire spread before the bus moved left and came to a halt. 

"There was a lot of smoke ... People started to throw up, scream, like in a horror movie," the 26-year-old from Presevo, Serbia, told Klan Macedonia.

"I managed to escape because I took the hammer from the bus and broke the glass. I took my fiancee and five more people."

His fiancee, Medina Lutfi, 25, says she also survived by jumping out of the window.

"I was sleeping and suddenly I heard fellow passengers crying and screaming ... there was black smoke, an explosion like a bomb ... I heard children crying, there were a lot of children," Lutfi said. 

Iliya Nikolov, a night guard at a nearby construction site, said he heard a "boom" and was among the first on the scene and called for emergency help. 

"We ran down the road ... and there was something burning in the guard rails. The fire was smaller in the beginning but then the back of the bus flamed up," Nikolov told bTV.