Indian authorities were on alert Saturday after a Bangladeshi tanker sank and dumped thousands of litres of oil into rivers of the Sundarbans, the world's largest mangrove forest.

The tanker collided with an empty cargo vessel Tuesday on the Bangladeshi side, spilling the oil into the protected area, which straddles India and Bangladesh and is home to rare Irrawaddy and Ganges dolphins.

Bangladeshis were attempting a big clean-up after environmentalists warned of an ecological "catastrophe" at the UNESCO World Heritage site where famed Bengal Royal Tigers roam.

On the Indian side, Pradip Vyas, director of the Sunderbans Biosphere, said: "There are no reports till now that the oil spill has reached the Indian part of the Sunderbans."

But Indian "wildlife officials have been deployed along the Sunderbans area bordering Bangladesh to check if the spill is spreading" as a precaution, he told AFP.

Bangladeshi villagers have been using sponges, shovels and even spoons to clean up the huge oil spill which has spread to a second river and a network of canals.

The oil has already spread over an 80-kilometre (50-mile) area.

Rescue vessels salvaged on Thursday the OT Southern Star 7 oil, which was carrying some 357,000 litres (77,000 gallons) of oil when it sank in the Shela river.

Rubayat Mansur, Bangladesh head of the New York-based Wildlife Conservation Society, who visited the trawler, said most of the oil seemed to have leaked out before it was salvaged and "only few hundred litres" of oil remained inside.

Local residents on the Bangladesh side reported seeing dead otters near river banks.

The Sundarbans forest sprawls over 10,000 square kilometres (3,800 square miles).

In 2011, the Bangladesh Inland Water Transport Authority opened a commercial shipping route through the Shela river despite opposition from environmentalists, an Indian wildlife official told AFP, on condition of anonymity.

Vessels plied this route between Kolkata, capital of India's West Bengal state, and Mongla Port in Bangladesh, the official said.

The Bangladesh government sent a ship carrying oil dispersants to the area, inside one of three sanctuaries set up for dolphins in the delta.

But environmentalists said the chemicals could harm the area's delicate ecology and Bangladesh authorities were debating whether to use the dispersants.

Meanwhile, the company that owns the oil tanker said it would buy oil that villagers have collected.

"It has no commercial value as it can't be used, but we are using the offer to encourage people so that the cleaning up process speeds up," said Rafiqul Islam Babul of the Padma Oil Co.