Bangladesh officials warn that an oil spill from a crashed tanker is threatening endangered dolphins and other wildlife in the massive Sundarbans mangrove region

Bangladesh officials warned Thursday that an oil spill from a crashed tanker is threatening endangered dolphins and other wildlife in the massive Sundarbans mangrove region, branding the leak an ecological "catastrophe".

The tanker carrying an estimated 350,000 litres (75,000 gallons) of oil collided on Tuesday with another vessel and partly sank in the Sundarbans' Shela river, home to rare Irrawaddy and Ganges dolphins.

Although officials are unsure how much oil has spilled, they warned the slick has spread to another river as well as a network of canals in the vast Sundarbans delta.

"The oil spill has spread over a 60 kilometre-long (37 mile) area in the Shela and Passur rivers," Amir Hossain, chief forest official of the Sundarbans, told AFP.

"It's a catastrophe for the delicate ecology of the Sundarbans. The oil spill has already blackened the shoreline, threatening trees, plankton, vast populations of small fishes and dolphins," Hossain said.

"The symptoms of environmental damage will be visible soon as the water quality has already been damaged," he added.

Authorities have launched a small-scale clean-up, but warned they lack the hardware and experience for a major effort. Navy boats and government officials were en route to salvage the tanker.

"We've not started any major clean-up efforts yet. In fact, the forest department doesn't have the technology to deal with this kind of disaster," said Hossain.

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

Spread over 10,000 square kilometres (3,900 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 straddling Bangladesh and India.

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. Fishermen are banned from making catches 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 fishes. 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.