Image copyright G. M. Masum Billah/WCS Image caption The oil spill has spread 40km around the rivers of the Sunderbans

The Bangladeshi 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.

Forestry officials said the spill happened in a sanctuary for dolphins including the rare Irrawaddy species.

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.

Heritage site

Image copyright G. M. Masum Billah/WCS Image caption The stricken tanker was tied to two other vessels to stop if from fully sinking

Image copyright AFP Image caption The Sunderbans is a Unesco world heritage site, home to rare animal and plant life

The tanker was reportedly on its way to deliver the oil to a power plant in Gopalganj when it was struck by another vessel.

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.