The accident occurred in one of the world’s largest mangrove forests, threatening rare animals.
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.
The oil tanker carrying more than 350,000 litres of bunker oil sank on Tuesday on a major river flowing through the Sundarbans after being hit by a cargo vessel.
Officials said yesterday the slick had spread over up to 70km of the Shela river, a major sanctuary for aquatic animals in the Sundarbans.
At least 20 canals connected with the Shela as well as another major river, Pashur, have also been affected.
The oil spill is threatening several types of animals including rare Irrawaddy dolphins, a senior official of the Bangladesh Forest Department said.
“The risk of damage to the biodiversity is high but we have yet to confirm any deaths of major animals including dolphins and crocodiles,” said Tapan Kumer Dey, chief conservator of forest wildlife.
The sunken oil tanker was salvaged on Thursday, more than 30 hours after it sank, and two of its six containers were badly damaged, said M Giasuddin, an official of the company that owns the vessel.
He said it was not clear whether all of the oil had spilled into the water.
Some news reports said more than 200,000 litres of oil had contaminated waters in the Sundarbans.
“Several teams are desperately trying to determine the immediate impact. We are closely monitoring the situation as this is a major disaster,” Dey said.
“We have spotted dolphins coming out of the water for air and going down again in some places,” he said.
“Crocodiles’ movement in the affected areas has been less after the disaster and we are trying to determine actually what happened to them.”
Dhaka’s Daily Star newspaper said oil has covered grasses and other plants on the banks of the rivers.
It quoted a local resident as saying that he spotted two dead animals, a monitor lizard and an otter.
Local villagers were using sponges and shovels to clean up the huge oil spill.
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 80km area.
Amir Hosain, the 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 said.
“We’re worried about its long-term impact, because it happened in a fragile and sensitive mangrove ecosystem.”