Ecologists believe the deaths were directly caused by the fire and release of hazardous chemicals while the Singapore-flagged X-Press Pearl burned for 12 days and sank last week off Sri Lanka's main port in the capital Colombo.

Government officials, however, say these causes were "provisionally" confirmed and the investigation is continuing.

The fire started on the ship on May 20 and dead marine species started washing ashore days later.

A ship manifest seen by the Associated Press said 81 of the ship's nearly 1500 containers held "dangerous" goods.

The Sri Lankan navy believes the blaze was caused by its chemical cargo, most of which was destroyed in the fire.

But debris including burned fibreglass and tonnes of plastic pellets have severely polluted the surrounding waters and a long stretch of the island nation's beaches.

The carcasses are being analysed in government laboratories.

"Provisionally, we can say that these deaths were caused by two methods - one is due to burns from the heat and secondly due to chemicals. These are obvious," said Anil Jasinghe, secretary of the environment ministry.

He refrained from giving an exact cause, saying "post-mortem analysis are still being conducted".

The sea off Sri Lanka and its coastline are home to five species of turtles that regularly come to lay eggs. March to June is the peak season for turtle arrivals.

Lalith Ekanayake, a marine and coastal ecologist, suspects "at least 400 turtles may have died and their carcasses may have sunk in the sea or drifted to the deep sea".

Sri Lanka plans to claim compensation from X-Press Feeders, the ship's owner, and already have submitted an interim claim of $US40 million ($A53 million).

Australian Associated Press