An explosion at a chemical factory in Slovenia killed five and left six injured, officials said Thursday.

A cistern exploded early Thursday, sparking a fire at the factory of resin supplier Melamin, according to fire brigade chief Leon Behin.

The factory is located in Kocevje municipality, some 60 kilometres south of Ljubljana.

"Unfortunately our fears have been confirmed," Melamin head Srecko Stefanic told reporters. The strength of the explosion "did not leave them any chance of survival", he said, in visible shock.

Officials earlier said four people were missing.

The victims were a worker and "four outside workers" at the company.

In addition, six workers were injured, including two who were hospitalised with serious burns, according to the hospital in Ljubljana treating them.

One of them is in a critical situation, Stefanic said.

The tragedy was "caused by a human error", he said, declining to give more information until the investigation has been completed.

Local authorities initially asked residents staying within a radius of 500 metres around the plant not to leave their homes and to close their windows as a precaution in case of toxic fumes.

The precautionary measure was lifted later Thursday after officials confirmed there had been no negative impact on the environment.

Photos showed columns of black smoke billowing from the factory, which supplies resins for paper, construction, wood, rubber and the lacquer industry among others.

Nearly 200 people work at the factory of the company, founded in 1954, according to its website.