Bangladesh's rapid development on the doorstep of the ecologically fragile Sundarbans mangrove forest means "environmental disasters" like this month's oil spill in the massive delta are increasingly likely, experts warn.

A cargo ship last week crashed into an oil tanker in thick fog in a river of the Sundarbans, whose intricate network of waterways is home to rare dolphins, endangered Bengal tigers and other animals.

Authorities failed to organise a proper clean up until four days after the sunken tanker spewed tens of thousands of litres of oil into a dolphin sanctuary -- ordering villagers and fishermen armed only with sponges and pans to scoop up the thick tar.

The Bangladeshi government opened up the delta in 2011 to large commercial vessels -- a decision environmental experts described as a "bomb waiting to explode".

The world's largest mangrove forest faces further threat from a range of projects underway to feed Bangladesh's booming economy, including a coal-fired power plant and a massive grain silo.

"The forest has lost half its cover in the last five decades. Now we've laid the groundwork to put the last nail in its coffin," Bangladesh's top independent wildlife expert, Mohsinuzzaman Chowdhury, said of the projects.

Chowdhury said he feared not only increased pollution from the projects but larger numbers of workers who would place greater stress on the delicate forest.

"Presently, around one million people are directly or indirectly dependent on the forest. But in the next decade, this number could grow to more than five million," Chowdhury said.

- World Heritage site -

The $1.7 billion power plant is being built 14 kilometres (nine miles) from the northern entrance of the delta to provide much-needed power to the impoverished but rapidly developing country.

Bangladeshi authorities came under fire over the 1,320-megawatt plant during a recent conference in Dhaka on efforts to save the world's tiger population.

National forestry chief Yunus Ali said his department initially "raised concern" over the plant being built on the banks of the Poshur river that flows into the forest.

"But the authorities have since adopted an environment management plan to mitigate any possible negative impact," he said.

A senior Sundarbans forest official cast "doubt" on the assessment, saying he was concerned waste from the tonnes of burnt coal would be dumped in the river when the plant finally becomes operational in 2018.

"Our main concern is waste and hot water management. Definitely, the plant would pump sludge in the Sundarbans' rivers. It will also spew thick dust, which will spread to the forest," the official said, speaking on condition of anonymity.

Local populations have long been allowed to hunt and fish in the Sundarbans, parts of which are UNESCO World-Heritage listed. Spread over 10,000 square kilometres (4,000 square miles), the delta is roughly two-thirds in Bangladesh and one-third in neighbouring India.

Bangladesh in 2011 allowed boats carrying everything from oil and cement to food to sail through the delta from the key southern port of Chittagong in the Bay of Bengal to the country's centre -- slashing cost and travel time.

A once-dormant port built at the far end of the Poshur river has seen increasing numbers of boats arrive in recent years as Bangladesh's economy records six percent annual growth.

A 50,000-tonne silo being built on the edge of the delta and set to open is also expected to bring increased ships transporting grain.

- 'Compromising conservation' -

Dead fish, crabs and other wildlife have washed up since the oil spill, which experts have called a disaster, although the overall impact is not yet known.

There were fears for the endangered Bengal tiger, several hundred of whom live in the Sundarbans, along with 300 species of birds and Irrawaddy dolphins.

A multi-year study by New York-based World Conservation Society spotted 6,000 dolphins in the waterways, making it the largest known home for the rare mammal.

Located at the mouth of the Ganges and Brahmaputra rivers between India and Bangladesh, the delta is also an important spawning ground for the latter's multi-billion dollar hilsha fish and shrimp industries.

Y.V. Jhala, a professor at the Wildlife Institute of India, said the power plant's impact plus increased shipping would be "disastrous" and would endanger a "whole range of aquatic animals and the Bengal tigers".

"By allowing ships to move through the forest, Bangladesh is compromising conservation ... (for the sake of) development," he said, suggesting a total ban on shipping through the forest and relocating the plant.

"The Sundarbans' ecosystem is too precious to be compromised. It has a great economic value and acts as a buffer against environment catastrophe from cyclones blowing in from the Bay of Bengal".

"Losing it will be a too big a loss for humanity."