The survivors of a bus inferno that killed at least 46 people scrambled to safety through a window after the vehicle caught fire and crashed in Bulgaria, officials say. Seven people escaped with burns from the vehicle, which was packed with tourists who were mostly Macedonian. The bus slammed into a barrier and went up in flames while returning from Istanbul in Turkey to North Macedonia. Twelve children were among the dead, including twin boys, aged four. Only four men and three women survived the disaster, which happened on a motorway south-west of the Bulgarian capital, Sofia, early on Tuesday. The scale of the loss has caused shock and grief in Bulgaria and North Macedonia, where three days of national mourning have been declared. Macedonian Prime Minister Zoran Zaev described the crash as "horrifying". "I met the injured in a hospital in Bulgaria. My heart broke when I was hearing the cry of a father who lost [his] child," Mr Zaev said. The PM said he had spoken to one of the survivors, who told him that passengers were asleep when the sound of an explosion woke them."They succeeded in breaking one of the windows and saved a few people. Unfortunately, the rest did not succeed," Mr Zaev told reporters. The cause of the disaster was not immediately clear. Bulgarian officials said the bus swerved off the motorway and tore away a 50m (164ft) section of the crash barrier. But it is not yet known if the bus caught fire before or after the crash. No other vehicles were involved in the accident. Pictures of the aftermath showed a section of the road where the crash barrier had been shorn off and the charred bus, gutted by the fire. The mayor of the nearby village of Pernik said the motorway was in poor condition on that section and there were often accidents in the area. The bus had been returning to the capital of North Macedonia, Skopje, from a weekend holiday trip to Istanbul. It was apparently travelling as part of a convoy of four buses and had stopped at a petrol station near Sofia about an hour before the accident. The other buses, which were a few minutes ahead, returned to North Macedonia safely.