An oil spill from a crashed tanker in Bangladesh is threatening endangered dolphins in the vast Sundarbans delta, officials warned Thursday, calling it an ecological "catastrophe".

The tanker was carrying an estimated 357,000 litres (77,000 gallons) of oil when it sank in the Sundarbans' Shela river, home to rare Irrawaddy and Ganges dolphins, after colliding with another vessel on Tuesday.

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.

Rescue vessels have now salvaged the tanker, but officials said the damage had 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 AFP.

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

Hossain said the oil had spread over a 60-kilometre-long (37-mile-long) area of the Sundarbans.

Spread over 10,000 square kilometres (3,800 square miles), 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.

Mansur said Bangladesh's coastal areas including the Sundarbans were the "largest known home" of the Irrawaddy dolphins.

"Irrawaddy Dolphins can be found in South East Asia. But their population size is very small compared to Bangladesh," said Mansur.

Bangladesh set up sanctuaries in the Sundarbans in 2011 after studies showed there were hundreds of endangered Irrawaddy and Ganges river dolphins there. Fishing is banned, but tankers and other boats are allowed to pass through.

- UN urges ban -

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.

"The tanker has been dragged to the shore. Two tanks housing 120,000 litres of oil remained intact, but the other four tanks with two-thirds of the ship's 357,000 litres of oil have already been spilled," shipping ministry spokesman Rafiqul Islam told AFP.

Islam said a government ship carrying 10,000 litres of oil dispersants would reach the spot within hours to begin a more substantial clean-up.

"The oil dispersants would break the spilled oils into droplets," he said, adding authorities have banned ships from using the Shela river channel until further notice.

Bangladesh's state-run petroleum corporation was also using buoys to restrict the slick, while local fishermen have been ordered to use nets to try to stop the oil entering small canals.

The United Nations urged the government to impose a complete ban on commercial vessels in the Sundarbans.

"The oil spill happened near the Chadpai Wildlife Sanctuary, the largest aquatic protected area declared by the Bangladesh government and home of two threatened dolphin species," said the head of the UN Development Programme in Bangladesh Pauline Tamesis.

"Global experience shows that this kind of incident has long-term environmental consequences."