COLOMBO: Sri Lankan authorities said Thursday they are preparing for the worst-case scenario of a possible oil slick from the burnt-out cargo ship sinking off Colombo's main harbour.

The Marine Environment Protection Authority readied oil dispersants, booms and skimmers in case of a leak from the MV X-Press Pearl, which has nearly 350 tonnes of oil in its fuel tanks.

The vessel, which burned for 13 days within sight of the island's west coast, has already caused the country's worst maritime environmental disaster, littering the beaches with huge volumes of plastic pollution.

But now that it is sinking, officials fear an even greater ecological crisis if the ship's oil leaks into the Indian Ocean.

"There is no oil leak from the ship yet, but arrangements are in place to deal with a possible spill which is the worst-case scenario," said navy spokesperson Indika de Silva.

An Indian coast guard vessel already in the area has equipment to deal with an oil slick before it could reach the beaches, according to the Sri Lankan navy, which has requested additional assistance.

The Singaporean operators of the MV X-Press Pearl said the vessel was continuing to slowly sink after efforts to tow it farther from the coast on Wednesday failed.

"X-Press Feeders... can confirm that the ship's aft portion is sitting on the seabed at a depth of about 21 meters (69 feet), and the forward section is settling down slowly," the company said in a statement Thursday.

Sri Lanka's navy said the bow of the vessel was still above the waterline as of Thursday morning.

"Even if the bow also hits the sea bed, still there will be a section of the upper deck and bridge sticking out of the water," navy spokesperson Indika de Silva told AFP.

He said there was no visible sign of bunker oil leaking from the ship.