A fire-stricken container ship that has already caused Sri Lanka's worst-ever maritime environmental disaster, was sinking fast on Wednesday, with nearly 1,500 containers and 25 tons of nitric acid on board, plus oil in its tanks.

Water submerged the MV X-Press Pearl's quarterdeck a day after firefighters extinguished a blaze that had been burning for almost two weeks.

Assisted by the Sri Lankan Navy, Dutch salvage firm SMIT was trying to tow the ship farther out to sea because if it sank at its current location off the port of Colombo, it would cause an even greater environmental problem, navy spokesperson Indika de Silva said.

'Stern submerged'

He said the stern had already sunk to the bottom at a depth of 22 meters (72 feet), around 6 nautical miles (11 kilometers) from the coast.

"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."

As a precautionary measure, authorities reimposed a fishing ban along 80 kilometers (50 miles) of the coastal area.

The fire erupted on May 20 when the MV X-Press Pearl was anchored about 9.5 nautical miles northwest of Colombo, awaiting to enter the port.

The navy believes the blaze was caused by the chemicals on board the Singapore-registered boat.

jsi/msh (AP, AFP, dpa)