(AFP) - A ship that ran aground off Mauritius leaking tonnes of oil into the ocean is cracking, the prime minister said Sunday, threatening an even greater ecological and economic disaster for the island nation.

More than 1,000 tonnes of fuel has seeped from the bulk carrier MV Wakashio into the azure sea off southeast Mauritius, befouling the coral reefs, white-sand beaches and pristine lagoons that lure tourists from around the globe.

But another 2,500 tonnes remain aboard the stricken vessel, which ran aground on a reef on July 25 but only started oozing from a crack in the hull in the past week.

Experts warn a further rupture could unleash a spill that will be beyond catastrophic for the fragile coastal ecosystem upon which Mauritius, and its economy, relies.

Prime Minister Pravind Jugnauth said response crews had managed to stymie the leak for now, but were bracing for the worst.

"The cracks have grown. The situation is even worse," he told reporters late Sunday.

"The risk of the boat breaking in half still exists."

Japan said Sunday it would send a six-member expert team to assist with what Mauritius has declared an unprecedented environmental emergency.

France also dispatched a naval vessel, a military aircraft and technical advisers from nearby Reunion Island after Mauritius appealed for international help.

Thousands of volunteers, many smeared head-to-toe in black sludge, have marshalled along the coastline, stringing together miles of improvised floating barriers made of straw in a desperate attempt to hold back the oily tide.

Mitsui OSK Lines, which operates the vessel owned by another Japanese company, promised Sunday to "make all-out efforts to resolve the case".

"We are terribly sorry," the shipping firm's vice president, Akihiko Ono, told reporters in Tokyo.

But some fear the damage is already done.

Aerial images show the enormity of the disaster, with huge stretches of crystal-clear seas around the marooned cargo ship stained a deep inky black.

Thick muck has coated mangrove forests and unspoiled inlets up and down the coastline, exacting irreparable harm and undoing years of painstaking conservation work, environmental activists say.

- 'Already too late' -

The slick has already begun drifting further up the coast, fanned along by strong winds and currents.

"I think it's already too late. If the ship breaks in two, the situation will be out of control," Vassen Kauppaymuthoo, an oceanographer and environmental engineer, told AFP.

"We’re talking about a major disaster that is progressing, and it’s getting more complicated hour by hour."

Pressure is mounting on the government to explain why more was not done in the two weeks since the bulker ran aground.

The opposition has called for the resignation of the environment and fisheries ministers, while volunteers have ignored an official order to leave the clean-up operation to local authorities, donning rubber gloves to sift through the sludge.

"People by the thousands are coming together. No one is listening to the government anymore," said Ashok Subron, an environmental activist at Mahebourg, one of the worst-hit areas.

"People have realised that they need to take things into their hands. We are here to protect our fauna and flora."

Police boarded the Japanese-owned but Panamanian-flagged Wakashio on Sunday and seized the ship's log book and black box as part of investigations into the disaster.

The bulker struck a reef at Pointe d'Esny, an ecological jewel fringed by idyllic beaches, colourful reefs, sanctuaries for rare and endemic wildlife, and unique RAMSAR-listed wetlands.

Mauritius and its 1.3 million inhabitants depend crucially on the sea for ecotourism, having fostered a reputation as a conservation success story and a world-class destination for nature lovers.

The spill is a double blow for tourist operators who had hoped foreign tourists could soon return to Mauritius. The Indian Ocean nation has no active cases of coronavirus, and had declared wary victory after a long stretch without any new infections.

But it also relies on its natural bounty for food and income. Seafarers in Mahebourg, where the once-spotless seas have turned a sickly brown, worried about the future.

"Fishing is our only activity. We don't know how we will be able to feed our families," one fishermen, who gave his name only as Michael, told AFP.

© Agence France-Presse