There was no explosion of the cistern loaded with liquefied petroleum gas that derailed on Saturday, causing a blast that killed 7 people and left 29 injured, Bulgarian firefighters' chief Nikolay Nikolov has said.

A total of twenty cisterns are on site after having gone off the tracks of a cargo train. Their content, propylene and LPG, is being drained out by special teams in a risky operation.

"There is no such fact of the tank exploding," Nikolov has told reporters in an emergency briefing as the operation is ongoing.

Nikolov has asked the public to wait for the conclusions of investigators working on the field.

However, so far it is evident that one of the tanks, which caused the deaths and damage, did not explode after derailing, but bumped into buildings and was damaged by the collision.

LPG then steamed out of it and created a cloud of gas in a big area in the center of Hitrino which was enflamed by a spark that could have even come from electricity, he has argued.

Nikolov has added that risk remains high in Hitrino, with drainage operations ongoing.

Additional equipment will be dispatched from Romania to help the cleanup activities as "Bucharest is closer [to Hitrino] than Sofia."