COLOMBO (AFP) - A burnt-out container ship that had already caused Sri Lanka's worst maritime environmental disaster was in imminent danger of sinking on Wednesday (June 2) with several hundred tonnes of oil still in its fuel tanks, the Sri Lankan navy said.

The MV X-Press Pearl, carrying hundreds of tonnes of chemicals and plastics, burned for 13 days within sight of the island's coast before rescue workers finally managed to extinguish the blaze on Tuesday.

The operator of the container ship, X-Press Feeders, confirmed in a press release that salvors had reported no visible flames aboard the vessel, though smoke was still emitting intermittently from cargo holds.

"An inspection team was able to board the vessel on the afternoon of Tuesday, June 1, and reported the engine room flooded. There are now concerns over the amount of water in the hull and its effect on the ship's stability," the press release said.

A huge amount of plastic debris has already inundated beaches, and the 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.

As tugboats on Wednesday began trying to tow the ship farther out to sea, navy spokesman Indika de Silva said it was "facing an imminent risk of going down".

Fisheries Minister Kanchana Wijesekera tweeted that the salvage company involved in the operation "has indicated that the vessel is sinking at the current location".

An AFP photographer at Sarakkuwa, just north of the Colombo port, said the vessel's stern was underwater.

An official involved in the mitigation efforts said earlier that local experts feared the vessel was unstable.

"The fire-fighting efforts also saw a lot of water sprayed onto the decks. Much of that water has settled in the stern, which is down by about a metre," the official told AFP.

"We can't pump that water out because it is contaminated with oil."

Sri Lankan President Gotabaya Rajapaksa on Tuesday ordered the ship moved to minimise potential coastal damage.

"(The experts') suggestion, then, was to take the vessel to the deep seas to minimise possible damage to the marine environment," his office said.

The Sri Lankan navy helped Dutch salvage company Smit to establish a tow connection with its tug to move the vessel away from the anchorage of the Colombo port, about 15km from the coast.

The inundation of microplastic granules from the ship's containers has already forced a fishing ban and prompted concern for the wildlife and environment.

Meanwhile, contractors continued working with the local authorities for a fifth day to dispose of any debris that had washed ashore.

The ship was also carrying 25 tonnes of nitric acid, which leaked and then caught on fire.

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

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

Mr Rajapaksa asked Australia on Monday to help evaluate the ecological damage to the island, one of the most biodiverse countries in South Asia.

Sri Lanka has launched a criminal investigation into the fire and the marine pollution.

Police said that the captain and chief engineer, both Russian nationals, as well as a third officer, have been questioned.

A court had ordered on Tuesday the passports of all three to be impounded, pending investigations.

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