Fifteen people died in a blast at a Bulgarian explosives decommissioning plant, officials confirmed Thursday, as workers complained of low pay and poor safety at the facility, which has seen several incidents in recent years.

"Thirteen men and two women perished. The blast was so powerful that it left craters," civil defence force director Nikolay Nikolov said.

At least 800,000 landmines were on site at the time according to the state prosecutor, who has launched an investigation into the accident.

Three female workers were also injured in the explosion, which rocked the mountainous area around the village of Gorni Lom in northwestern Bulgaria Wednesday evening, residents told state television.

The women were taken to hospital and were not in critical condition.

Nikolov said "human error" was the most probable cause of the blast, which razed several buildings at the small plant.

The devastation was so complete that "no traces whatsoever of bodies could be seen" at the site, he added, noting that police forensic workers were continuing to search the area.

According to the plant's management, 12 workers and three of the factory chiefs were inside the facility at the time of the blast, which happened as Greek landmines were being taken apart.

Local resident Dimitar Dimitrov, 59, told AFP: "I used to work there and I sneaked inside the area long before the engineers arrived. Everything was razed to the ground."

Among the locals, many of whom lost friends and relatives, the disaster did not come as a surprise.

Safety rules were ignored at the plant, outdated machinery was used, and workers were paid less than the Bulgarian minimum salary of 340 leva (170 euros, $220), they complained.

Dimitrov said his 26-year-old son Vasil, who was among the victims, was paid 240 leva and working conditions were bad.

"My son looked for work for three and a half months and was finally forced to go there," the tearful mother of another victim told private bTV television.

"I begged him to refuse this dangerous job but he had a family to feed."

"This is what happens when safety rules are not observed. I used to work at the plant when it was state-owned -- there were iron rules and no accidents," another elderly woman added.

Bulgarian President Rosen Plevneliev angrily said Thursday: "Innocent human lives were lost because of the arrogant breaching of instructions and rules in state institutions governing work with explosives."

"Sadly, this is also not a precedent but the latest deadly case," he added.

- Deadliest of several incidents -

Explosions in 2007 and 2010 at the privately owned facility, which employs 50 people, injured six people, and labour ministry inspectors last month uncovered numerous problems and poor "technical discipline" there, for example with rubbish barring escape routes.

Two people were also killed in two separate accidents in 2006 and 2007, the country's main prosecutor said, without specifying the type of accident. The plant's chief executive was charged at the time but found not guilty.

Bulgaria, the EU's poorest country, has large amounts of ageing ammunition from the communist era that has been awaiting disposal, and similar incidents have multiplied in recent years.

None however has been as deadly as this one.

Though Bulgaria is preparing to vote in general elections on Sunday, all political parties cancelled their closing rallies for Friday when the country will hold a national day of mourning.