Kočevje - A huge explosion at the Melamin chemical plant in Kočevje, south Slovenia, on Thursday morning is believed to have killed five people. They remain unaccounted for, presumably buried under the rubble, making this the worst industrial fire in modern Slovenian history.

"The five missing persons have probably perished because the explosion was so strong there was no chance they survived," the company's director Srečko Štefanič said.

Two more people are in hospital with severe burns and one of them is still fighting for their life, he said.

The explosion occurred when epichlorohydrin, a volatile substance that was being transferred from a tank truck to an on-site cistern, burst into flames, according to Štefanič.

Štefanič said the precise cause of the blast remained unclear but it was likely the result of human error, adding two compounds appeared to have come into contact.

Only one of the missing persons is a Melamin employee, the rest are contractors who were working on the site at the time.

The plant, which produces plastic materials, paints, varnishes and other coatings and chemical products, is located not far from the centre of Kočevje and the 8:30am blast was heard dozens of kilometres away.

While a handful of people were treated for smoke inhalation, the blast did not cause damage in the community.

Firefighters have completed their work and only about a dozen remain on-site as a precaution, said commander of the Kočevje fire brigade and head of the firefighting operation Leon Behin.

The recovery of the bodies will not be able to start before the rubble cools down, presumably tomorrow.

The recovery work is likely to be hampered by unstable steel structures, which are at risk of collapsing after the fire, he said.

"This was a tragic explosion of extreme proportions," according to him.

While the immediate impact of the blast is severe, the economic aftermath could be painful as well given Melamin's role as a major employer in a region that was long depressed and only started to economically recover a few years ago.

Štefanič, the director, said it will be very difficult to restore production since a lot of equipment was damaged in the blast, including high-tech gear.

The plant is likely to stand still for a while. He said the company could only be salvaged if its partners help it.