Efforts to clean up started Friday after an oil tanker collided with another boat in the Sundarbans national park on Tuesday.
The tanker was carrying an estimated 357,000 liters (77,000 gallons) of oil when it sank. Most of the oil had already leaked out by the time the tanker was dragged from the area on Thursday.
The oil has spread over an 80 kilometer (50 mile) area of the Sundarbans, a UNESCO world heritage site made up of rivers, canals and mangrove forest which is home to rare Irrawaddy and Ganges dolphins.
"This catastrophe is unprecedented in the Sundarbans and we don't know how to tackle this," Amir Hosain, chief forest official of the Sundarbans, told the news agency AFP. "We're worried about its long-term impact, because it happened in a fragile and sensitive mangrove ecosystem," he said.
Local fishermen deployed by the authorities are using foam, sacks, sponges and nets to collect the oil and prevent it spreading further into the forest, located around 360 kilometers (224 miles) southwest of Bangladesh's capital, Dhaka.
The government has sent a ship carrying oil dispersants to the site, but environmentalists are concerned that these chemicals could harm the delicate ecology of the area.
The tidal rivers in the Sundarbans regularly swell, covering the low lying sections of mangrove forest. This has caused the oil pollution to spread, according to Faruk Ahmed, an environmental expert at the nearby Khulna University. "The spilled oil has been deposited inside the forest, threatening the flora and fauna of the ecosystem," he said.
Pauline Tamesis, country director of the United Nations Development Program in Bangladesh, also expressed alarm over the oil spill, calling for the government to stop commercial shipping routes through the mangrove forest.
Currently, boats are allowed to pass through the area, although fishing is banned.
lvw/tj (AFP, dpa)