International experts boarded an oil tanker stricken off Sri Lanka to begin salvage operations, the navy said Thursday, as firefighters looked to contain a large oil slick in the Indian Ocean.

A huge week-long blaze aboard the New Diamond was finally extinguished Wednesday and though 270,000 tonnes of crude oil were unaffected officials said, diesel fuel had leaked into the water, creating a significant and expanding slick.

"The firefighting is over," said navy spokesman Captain Indika de Silva.

"We are taking a step back and now it is up to the owners to decide how they want to take it from here."

Officials said the fuel slick had reached 2 kilometres (1.2 miles) in length after doubling in size overnight, as the tanker drifted around 20 kilometres north of its Wednesday location.

The tanker is tied to a tug and keeping a distance of just under 100 kilometres from Sri Lanka's shoreline.

On Wednesday, the Indian Coast Guard (ICG) said it was deploying an aircraft to spray chemicals and minimise the impact of the fuel that leaked from the tanker.

A huge week-long blaze aboard the New Diamond was finally extinguished Wednesday
A huge week-long blaze aboard the New Diamond was finally extinguished Wednesday Sri Lankan Air Force / -

The salvage experts who boarded the vessel reported that toxic gases had filled the stern section of the ship and that they will be removed using air blowers, Sri Lanka's navy said.

A powerful tug boat commissioned by the Dutch salvage company SMIT arrived from Singapore on Thursday to boost their efforts, it added.

The tug has specialised equipment to drain the sea water -- used to flood the engine room during the firefight -- and remove the gases.

It was not immediately clear what the salvage company would do with the Panamanian-registered tanker, but Sri Lankan authorities have asked them to tow it away from the island's waters.

The New Diamond issued a distress signal a week ago while passing Sri Lanka on its way to the northeast Indian port of Paradip when a boiler exploded killing a Filipino crewman. The remaining 22 crew were rescued.

Both Sri Lanka's navy and the ICG said the tanker's cargo of 270,000 tonnes of crude oil were still on board despite the fire that ravaged the vessel.

Sri Lanka also plans to seek compensation for the diesel spill and recover the cost of responding to the fire and preventing a break up of the 330-meter (1,100 foot) tanker.

Oil Spill