Bangladesh on Monday banned boats from sailing through a key southwestern river after a ship loaded with coal capsized, threatening the sanctuaries of rare dolphins in the world's largest mangrove forest.

The authorities imposed the ban after the ship sank in the Shela river on Saturday carrying over a thousand tonnes of coal, raising fears for two sanctuaries of endangered Irrawaddy and Ganges river dolphins and the delicate ecology of the Sundarbans forest.

"We have decided to ban all types of naval movement at the Shela river indefinitely," shipping secretary Ashok Madhob Roy told AFP, saying the ships would be rerouted to another channel on the edge of the forest.

Bangladesh suspended cargo boat transport through the same river in 2014 after a catastrophic oil spill that damaged the Sundarbans and triggered concerns for the forest's dolphins and other endangered animals including Bengal tigers.

But the suspension was lifted under pressure from local trade groups, who have said the Shela river must be open for cargo vessels to ensure the smooth supply of industrial goods and food-grains to the southwestern region.

Officials said the hull of the vessel, which sank carrying more than 1,200 metric tonnes of coal and is yet to be salvaged, was cracked.

"The sunken coal could pose grave threat to the aquatic biodiversity of the Sundarbans," forest conservator Zahir Uddin Ahmed told AFP.

"If the coal contains too much sulphur and if it dissolves into the water, then it is a dire concern," Ahmed said.

"The effect of oil spillage from the ship could also be damaging."

Spread over 10,000 square kilometres (3,900 square miles), the Sundarbans is the world's largest mangrove forest and the core part of it is a UNESCO-listed World Heritage Site.

The forest is home to scores of endangered Bengal tigers, spotted deers, fresh-water crocodiles and rare dolphins.

The pristine mangrove forest, said to be the South Asian nation's largest protection against tsunamis and cyclones, is already facing unprecedented human and industrial encroachment and poaching by gangs of sophisticated pirates.

This month thousands of Bangladeshi environmental activists joined a 250-kilometre (155-mile) long-march to the country's southwest to protest the construction of two coal-fired power plants near the Sundarbans.

Experts fear the industrial waste of the 1,320-megawatt plant, a $1.7 billion joint venture of India and Bangladesh, and another one by a private company would destroy the forest.