At least four people have lost their lives as a train exploded after going off the tracks in northeastern Bulgaria, police say.

An unknown number of people have been injured.

Medical officials, quoted by public broadcaster BNT, say at least 27 have been hospitalized, many of them heavily injured. Thirteen of them have left the hospital in the nearby town of Shumen with minor injuries, while others have been transferred to bigger medical institutions in Varna, at the Black Sea. Four have been admitted to the Naval Hospital in Varna.

At 05:40 local time (EET), cisterns loaded with propylene liquefied petroleum gas and mounted on a cargo train exploded as the train left the tracks near Hitrino, not far from Shumen, while moving along the Ruse-Varna railroad.

All ambulances from the nearby towns of Razgrad, Shumen and Targovishte have been deployed to the site of the incident, the head of Bulgaria's firefighters department, Nikolay Nikolov, has told Focus News Agency.

At least 20 buildings, including the railway station, have been destroyed by the blast. New explosions are not expected, but the immediate surroundings of the site are being evacuated, Nikolov said.

The reasons for the crash have been not immediately clarified.

Outgoing Bulgarian Prime Minister Boyko Borisov is traveling to the site. He has urged citizens to donate blood to address the shortage at the Shumen hospital.