A fire broke out aboard the MV X-Press Pearl, which was carrying 25 tonnes of nitric acid and a huge amount of plastic raw materials, as it was about to enter Colombo harbour on May 20.

There were no visible flames aboard the vessel mid-Monday, but smoke was rising from the rear section, its operator and officials said.

Firefighters were still working at the site, they said, adding that the stricken ship was still too hot to be boarded.

Detectives interviewed the vessel's skipper and chief engineer -- both Russians -- and its chief officer, an Indian, at a hotel where they have been quarantined, police said.

"We have questioned three members of the crew so far and the investigations are ongoing," a police spokesman added.

"Select seafarers have today been helping local police with their enquiries into the fire and are cooperating with investigators," the vessel's operators, X-Press Feeders, added in a statement.

"We will respect this process and not publicly discuss operational details until the investigation has been completed."

The intense flames destroyed much of the cargo, some of which fell into the Indian Ocean.

Tonnes of microplastic granules from its cargo of nearly 1,500 containers have inundated Sri Lanka's famed beaches, forcing a fishing ban and sparking fears of ecological devastation.

The Marine Environment Protection Authority (MEPA) chief Dharshani Lahandapura said they were still assessing the ecological damage, but believed it was the "worst ever in my lifetime".

MEPA said the captain of the vessel had known about a nitric acid leak on May 11, long before the vessel entered Sri Lankan waters.

Authorities are moving to sue the owners of the vessel and its crew and claim damages from insurers.

The three-month-old ship was heading to Colombo from Gujarat, India.

It had previously visited Qatar and Dubai and was due to go to Malaysia and Singapore after calling at Colombo.