An oil spill from a crashed tanker in Bangladesh is threatening endangered dolphins and other wildlife in the vast Sundarbans delta, officials warned, branding the leak an ecological "catastrophe".

The tanker was carrying an estimated 357,000 litres of oil when it sank in the Sundarbans' Shela river, home to rare Irrawaddy and Ganges dolphins, after colliding with another vessel on Tuesday.

Rescue vessels have now salvaged the tanker, but officials on Thursday said the damage had already been done as the slick had spread to a second river and a network of canals in the Sundarbans, the world's largest mangrove forest, which straddles India and Bangladesh.

"It's a catastrophe for the delicate ecology of the Sundarbans," the area's chief forest official Amir Hossain told the AFP news agency.

"The oil spill has already blackened the shoreline, threatening trees, plankton, vast populations of small fishes and dolphins."

Hossain said the oil had already spread over a 60km-long area of the Sundarbans.

Spread over 10,000 square kilometres, the Sundarbans is a UNESCO-listed World Heritage Site and home to hundreds of Bengal tigers.

The delta comprises a network of rivers and canals.

No coordinated efforts

The accident occurred inside one of three sanctuaries set up for the dolphins, said Rubayat Mansur, Bangladesh head of the New York-based Wildlife Conservation Society.

The three areas were declared dolphin sanctuaries in 2011 after studies found they are home to some 6,000 of the animals. Fishing is banned there, but tankers and other boats are allowed to pass through.

Speaking to AFP from the accident site, Mansur labelled the spill a "national disaster" and accused authorities of not doing enough to contain the damage.

"There are no coordinated efforts to tackle the disaster. The air has become toxic and we got news from fishermen they've seen dead fish. Crabs, which make up the largest single group in the forest are facing the biggest threat," he said.

"And if crabs are hit, the dolphins and tigers will be affected. Dolphins will find it very difficult to breathe this foul air," he added.

Authorities have launched a small-scale clean-up, but warned they lack the hardware and experience for a major effort.