Sri Lanka's worst maritime environmental disaster took a turn for the worse as a burnt-out ship filled with chemicals sunk on Wednesday with several hundred tonnes of oil still in its fuel tanks.
The MV X-Press Pearl, carrying hundreds of tonnes of chemicals and plastics, blazed for 13 days within sight of the island's coast before rescue workers extinguished the flames on Tuesday.
A huge amount of plastic debris inundated beaches. 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 tugs on Wednesday began trying to tow the ship further out to sea, navy spokesperson Indika de Silva said it was slowly sinking.
"The stern of the ship is under water, the water level is above the deck," Mr de Silva said.
"The ship is going down."
Some oil was already visible near the beaches of Negombo, about 40 kilometres from Colombo, an AFP photographer said, although it was not clear if it was from the stricken ship.
Mr De Silva said the navy helped Dutch salvage firm Smit board the vessel and establish a tow connection after earlier attempts failed in bad weather.
"The ship will be towed as far away from the coast as possible before it goes down completely," he said.
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 official said earlier that local experts feared the vessel was unstable.
"The firefighting efforts also saw a lot of water sprayed on to the decks. Much of that water has settled in the stern," the official said.
Authorities had planned to pump contaminated water from the ship on to barges but the operation has been abandoned.
The navy said an Indian coastguard vessel in the area had the equipment to deal with an oil slick if necessary.
The spread of microplastic granules from the ship's containers has already forced a fishing ban and prompted concern for wildlife and the environment.
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".
President Gotabaya Rajapaksa asked Australia on Monday to help to evaluate the ecological damage to the island, one of the most biodiverse countries in South Asia.
Sri Lanka has launched a criminal investigation.
The authorities believe that the fire was caused by a nitric acid leak that the crew apparently knew about from May 11, nine days before the blaze started.
Police said that the captain and chief engineer, both Russian nationals, and a third officer have been questioned.
A court ordered the impounding of the passports of all three pending investigations.
The ship was heading to Colombo from Gujarat, India, when the blaze started, having previously visited Qatar and Dubai.
