Experts fear the MV X-Press Pearl is in imminent danger of sinking.

A burnt-out container ship that has already caused Sri Lanka's worst maritime environmental disaster was sinking fast Wednesday with nearly 300 tonnes of oil still in its fuel tanks.

The MV X-Press Pearl, carrying hundreds of containers of chemicals and plastics, burned for 13 days just outside the Colombo harbour before rescue workers finally extinguished the blaze on Tuesday.

Dutch salvage firm SMIT attempted to pull the still-smoking wreckage of the 31,600-tonne vessel further into deeper waters on Wednesday but the effort stalled, navy spokesman Indika de Silva told AFP.

He said the stern had sunk to the bottom at a depth of 22 meters (72 feet), six nautical miles (11 kilometres) from the coast of Pamunugama, just north of Colombo.

"The bow is still afloat, but the stern is submerged and resting on the sea bed," de Silva said. "As a result, the towing of the ship has been stopped."

A huge amount of plastic debris has already inundated beaches, and authorities now fear an even greater disaster should the 278 tonnes of bunker oil and 50 tonnes of gas in the Singapore-registered ship's fuel tanks leak into the Indian Ocean.

Some oil was already visible near the beaches of Negombo, about 40 kilometres (25 miles) from Colombo, an AFP photographer said, although it was not clear if it was from the stricken ship.

International shipping academic and lawyer Dan Gunasekera said there were no immediate signs of bunker oil leaking, but said divers may have to be used to pump it out safely.

Navy spokesman de Silva said an Indian coastguard vessel was in the area with specialised equipment to deal with any oil slick.

An official involved in the operation said some of the vast amount of water sprayed on the fire to put it out had made the vessel unstable.

"Much of that water has settled in the stern," the official told AFP.

The fire has already caused Sri Lanka's worst marine ecological disaster.

Oil disaster

Officials believe the blaze destroyed most of the nearly 1,500 containers onboard.

The inundation of microplastic granules from the cargo has already forced a fishing ban and prompted concern for wildlife and the marine environment.

Marine Environment Protection Authority chief Dharshani Lahandapura said the ecological damage was still being assessed, but he believed it was the "worst ever in my lifetime".

President Gotabaya Rajapaksa asked Australia on Monday to help evaluate the ecological damage to the island, one of the most bio-diverse countries in South Asia.

Call to sue

The head of Sri Lanka's Roman Catholic church, Cardinal Malcolm Ranjith, called for legal action against the authorities for allowing the ship into local waters.

He said that the livelihoods of thousands of fishermen, most of whom were from the Catholic minority, was now under threat.

Sri Lanka has already launched a criminal investigation.

Authorities believe the fire was caused by a nitric acid leak which the crew apparently knew about from May 11, nine days before the blaze started.

The 25-strong crew was evacuated last week. One was admitted to hospital with minor injuries.

Police said three officers from the ship, two Russians and an Indian, had been questioned and their passports had been impounded.

The ship was heading to Colombo from Gujarat in India when the blaze started, having previously visited Qatar and Dubai where the containers of 25 tonnes of nitric acid were loaded.