Wednesday's blast at the Midzhur explosives plant resulted in the death of 13 men and 2 women, Interior Ministry officials say.

Nikolay Nikolov, head of Chief Directorate Fire Safety and Civil Protection at the ministry, has thus confirmed previous estimates that three employees and three management executives were inside the factory at the time of the incident.

"Hopes to find survivors after yesterday's explosion... have vanished," Nikolov told journalists.

In his words, the blast at 16:58 EEST (14:58 GMT) was exceptionally powerful and left two huge craters at the site of the incident.

Two of the buildings and parts of the plant's main shed have been burned to the ground, he explained, adding authorities will go on with the monitoring of the area.

A certain "human error" caused the incident, Nikolov declared, but stressed he did not know what it was precisely.

Land mines delivered from Greece were in the factory at the time of the explosion, but it is unclear if they are anyhow related to the events.

For hours the government had refused to be more specific about the death toll as an operation of a specially equipped team unfolded.

Videx, the company owning the Midzhur plant, is specialized in the production of explosive materials and hand grenades.

Its output is sold mostly in neighboring or Southeast European countries, with Greece, Macedonia, Serbia, Albania, Georgia and Israel being among the export destinations.

Earlier police entered the company's headquarters, seizing documents related to the plant.