Bangladeshi villagers using sponges, shovels and even spoons worked Friday to clean up a huge oil spill in a protected area that is home to rare dolphins, after environmentalists warned of an ecological "catastrophe".

Thousands of litres of oil have spilt into the protected Sundarbans mangrove area, home to rare Irrawaddy and Ganges dolphins, after a tanker collided with another vessel on Tuesday.

The government has sent a ship carrying oil dispersants to the area, which is inside one of three sanctuaries set up for the dolphins.

But environmentalists say the chemicals could harm the delicate ecology of the Sundarbans, a UNESCO world heritage site.

As authorities debated whether to deploy the dispersants, the company that owns the stricken oil tanker said it would buy up the oil that local 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 Company.

"Villagers including children are going out onto the river in boats to collect the oil floating on the water using sponges, shovels and spoons," he said.

"Then they are putting it in small ditches on the river banks and our employees are buying it."

The head of the local port authority earlier told reporters that fishermen would use "sponges and sacks" to collect the spilt oil, which has already spread over an 80-kilometre (50-mile) area.

Amir Hosain, chief forest official of the Sundarbans, admitted that authorities were unsure about the best course of action.

"This catastrophe is unprecedented in the Sundarbans and we don't know how to tackle this," he told AFP.

"We're worried about its long-term impact, because it happened in a fragile and sensitive mangrove ecosystem."

- Damage already done -

Rescue vessels have now salvaged the tanker, which was carrying an estimated 357,000 litres (77,000 gallons) of oil when it sank.

But officials say the damage the has already been done as the slick has spread to a second river and a network of canals in the Sundarbans, the world's largest mangrove forest, which straddles India and Bangladesh.

Rubayat Mansur, Bangladesh head of the New York-based Wildlife Conservation Society, said most of the oil appeared to have already leaked out of the tanker before it was salvaged.

"I visited the sunken trawler this morning. Only few hundred litres of oil remain inside, so almost all the oil has spilled into the Sundarbans," he said.

Mansur said oil dispersants were "not appropriate for the mangrove ecosystem" and urged local villagers to help collect the oil from nets that have been placed in the river to contain its spread.

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 that there were hundreds of endangered Irrawaddy and Ganges river dolphins there.

Fishing is banned in the area, but tankers and other boats are allowed to pass through.

The Irrawaddy and Ganges dolphins are both on the warning "red list" of the International Union for Conservation of Nature (IUCN), which says numbers are falling.