A dead dolphin has been found in Bangladesh's protected Sundarbans delta following an oil spill which has spread over several hundred square kilometres, heightening fears for the area's rare wildlife.

The Dhaka Tribune newspaper published a photo of the mammal on Sunday, saying it was found floating in a canal off the Shela river where a crashed tanker has leaked thousands of litres of oil.

It was not immediately clear whether the dolphin was killed by the spill which officials said now covered up to 350 square kilometres (135 square miles) of the vast Sundarbans mangrove delta straddling India.

"According to latest information, the furnace oil has spread to an estimated 300 to 350 square kilometre area inside the Sundarbans," said a forest department statement late Saturday, updating an earlier estimate from officials of 80 square kilometres.

The tanker collided with another vessel and sank on Tuesday in an area of the Sundarbans set up as a sanctuary for hundreds of endangered Irrawaddy and Ganges river dolphins.

Forest authorities said an autopsy would be conducted on the dolphin, reportedly found about 25 kilometres (15 miles) from the sunken tanker, to determine if it died from the spill.

"We've sent a team of forest rangers to find out about the dead dolphin. We've to conduct a post mortem to know for sure what caused its death," Amir Hossain, chief forest officer of the Sundarbans, told AFP.

Shipping minister Shahjahan Khan has played down the impact of the spill, saying on Saturday there would be "no major damage".

But a senior forest official and other experts have called the accident an ecological "catastrophe".

Some 300 fishermen in 100 boats have been tasked with scooping up the oil using pots, pans and sponges, while villagers were cleaning up the river's banks, the forest department statement said.

"On Saturday, these boats collected 18,000 litres of spilled oil, which is nearly four times the amount collected by villagers," Hossain also said.

"The manual clean-up process is suitable for this ecosystem. We're already getting results. Our target is to mitigate the damage as much as we can," he added.

A state-run oil company is buying back the collected oil as an incentive to hasten the clean up.

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

The delta comprises a network of rivers and canals, where rare aquatic animals including the dolphins and freshwater crocodiles are found.

Monirul Khan, a zoology professor of Dhaka's Jahangirnagar University, warned the spill could be "the largest catastrophe to have happened in this fragile mangrove ecosystem".

"It's going to cause a long term damage to the forest. The impact would be far worse than the effect of the Cyclone Sidr," he said on Sunday, referring to a 2007 storm that killed nearly 3,000 people and tore up the delta.