The massive fire on board the container ship Sea Elegance, lying off Durban, was probably caused by an explosion in a 20-ton container of undeclared hazardous cargo that self-ignited, the South African Maritime Safety Authority (Samsa) said on Tuesday.

Samsa's Bill Dernier said it appeared that the crew of the Singapore-based container ship had been unaware they were carrying hazardous cargo.

He said the fire, which started on Saturday, was still smouldering, but under control.

"The fire started in a container in the lower part of the hold. We're convinced it was caused by an explosion of hazardous cargo that was not properly declared," Dernier said.

The container held tons of calcium hypochlorite, classed as category five in the International Maritime Organisation's dangerous goods code.

Category one is the least hazardous and category 10 the most hazardous. In terms of international maritime law, hazardous cargo has to be declared, specially packed and correctly stowed to reduce risk to crew and the vessel.

"We're told that calcium hypochlorite is liable to decompose at elevated temperatures which may lead to fire or an explosion. This container was stored right next to the engine room bulkhead, which is a hotspot and where it should not have been.

"It was stored next to a herbicide, Atrazine. When the container exploded, it may have opened up the Atrazine container, which burns like nobody's business. There were also rolls of paper, tyres and plastic in number six hold," Dernier said.

He said when Samsa officials investigated the fire, they had read the ship's manifest, which showed no hazardous cargo.

The officials then called for the ship's packaging declaration, which showed that it was carrying a container of calcium hypochlorite. It appears that this had not been declared as hazardous cargo.

"As far as the ship's crew was concerned, they had loaded general cargo only."

Dernier said Samsa was conducting a preliminary investigation into the fire because a Sri Lankan crew member, MMK Senanayake, had gone missing in the blaze and was presumed dead. The other 23 crew have all been accounted for.

Gareth Hughes of Foreshore Shipping in Durban, agents for the vessel, said on Tuesday his company had been instructed by the ship's owners not to comment on the cause of the fire. He had also been instructed not to name the ship's owners.

The ship is at anchor off Durban, attended by Smit Marine's John Ross tug. She will not be allowed into port until the authorities are satisfied she poses no risk, Dernier said.

The fire had not caused any pollution or oil spills.

Environment Minister Valli Moosa condemned the non-declaration of hazardous cargo, which could have a serious impact on people and the marine environment. - Environment Writer