The number of fatalities after the incident in Hitrino in northeastern Bulgaria has increased to seven, authorities say.

More than two dozen people have been left injured.

On Saturday morning, cisterns full of liquefied petroleum gas and propylene went off the rail tracks and crashed into buildings in Hitrino, with one of them subsequently exploding.

Up to 20 buildings have been damaged, some seriously, as the rail line via Hitrovo passes through the village's central area.

Monday will be a national mourning day.

It is not yet clear what caused the incident.

Search operations for survivors under the debris of destroyed buildings have been suspended, with more than a dozen having been pulled out of the ruins alive, firefighters' chief Nikolay Nikolov has said. 

"There will be more victims for sure," outgoing Bulgarian Prime Minister Boyko Borisov earlier said after arriving to the site of the explosion.

Among those hospitalized, many have sustained burns on their faces and limbs. Doctors say at least three of them are in critical condition.

Blood shortages that were initially reported have now been addressed, the head of the hospital in the nearby town of Shumen says.

"The damage is big after the heavy blast... The train driver said part of the composition split from the rest of the waggons," Borisov said when asked whether the driver or the transport company were to blame. He has declined to comment, explaining the investigation is being overseen by Sotir Tsatsarov, Bulgaria's Chief Prosecutor.

Funding will be disbursed to repair the damage, but the primary concern is what to do with residents whose homes were distroyed, the Prime Minister in resignation has added.

Nikolay Nikolov explained that emergency teams arrived at 06:00 (EET) local time, just 18 minutes after having been alerted, with the closest emergency center being 20 km away.

Forced evacuation was applied to the immediate surroundings, inhabited by more than 40 families, despite the fact there is "no direct danger to the life and health of residents of Hitrino". The aim is to mitigate all risk, Nikolov made it clear. There is still danger for emergency teams as new blasts may follow.

However, some of the locals criticized firefighters for acting too slowly, a claim Health Minister Petar Moskov has denied.

A man lashed out in front of the cameras, accusing firefighters for arriving late on the site. His wife died in the bakery as the latter was caught in flames after the explosion.

Hours later, the man claims it took firemen more than an hour to arrive.

President-elect Rumen Radev also arrived at Hitrovo on Saturday afternoon.

What matters in the aftermath is to let investigators do their job and for authorities to provide solutions to residents who were injured and whose property was lost or damaged.