Media playback is unsupported on your device Media caption Footage shows Bangladeshi villagers scooping oil from rivers

Environmental authorities in India are on alert after thousands of litres of oil spilled into a nature reserve in neighbouring Bangladesh.

Conservation chief Pradeep Vyas said "precautions" were being taken in West Bengal's coastal region.

Oil leaked into waterways in the Sundarbans area after a collision involving a tanker three days ago.

Officials say it is already harming the region's wildlife, which includes two endangered dolphin species.

"We are taking all precautionary measures," Mr Vyas, additional director of India's Sundarban Biosphere Reserve, told Reuters on Friday.

Footage shows Bangladeshi villagers scooping oil from rivers using pots and pans and sponges, and birds covered in black liquid in the Sundarbans mangrove forest in south-west Bangladesh.

The oil spilled after a tanker carrying 350,000 litres (77,000 gallons) collided with another vessel, spreading a sheen across 60km of waterways.

The Sunderbans is a Unesco heritage site, home to many rare species.

The oil leaked into the river at a sanctuary for rare dolphins.

Environmentalists have expressed concern that the impact on wildlife could be disastrous.

But government officials say they still cannot give an assessment of the likely damage.

Image copyright AFP/World Conservation Society Image caption Villagers have been asked to help the government clear the spill

Image copyright AFP Image caption A thick, black sheen spread in the days after the tanker crash, covering some 60km of waterways

The UN on Thursday called for a complete ban on all commercial vessels using the waterways of the Sunderbans.

The Bangladeshi navy initially sent four ships to deal with the spill, and planned to use chemicals to disperse the oil.

But it was unclear whether the chemicals could be used, amid fears they could cause further damage.

"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 AFP news agency.

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

Image copyright G. M. Masum Billah/WCS Image caption The stricken tanker was pulled from the river on Thursday

Image copyright AFP Image caption Officials are still unclear about the extent of the damage from the spill

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

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.