The Bangladesh navy has sent four ships to deal with an oil spill from a crashed tanker that is threatening a rare dolphin sanctuary. The tanker, believed to be carrying 350,000 litres (75,000 gallons) of oil, and another vessel collided early on Tuesday in the Sundarbans region.
The government has lodged a legal case against the owners of the two vessels.
Officials told the BBC the oil had spread along a 40km (25 mile) section of the Sela river.
It is not clear how much oil spilled out of the vessel, but some officials were quoted as saying all 350,000 litres might have gone into the water.
Two navy ships are already at the scene, and two others are on their way. They are carrying chemicals to try to separate the oil from the water. But Bangladeshi officials say they have little experience of dealing with such problems. The Sunderbans, a Unesco world heritage site, is a vast river delta on the northern shore of the Bay of Bengal.
Its mangroves and rivers are home to a vast array of plant and animal life, much of it unique to the region.
The government declared areas in the southern Sunderbans to be a dolphin sanctuary in 2011, after research suggested some 6,000 Irrawaddy dolphins lived in the area.