Large cracks have reportedly appeared in the hull of a cargo ship leaking oil in Mauritius, prompting the prime minister to warn it may "break in two".
The MV Wakashio, believed to have been carrying 4,000 tonnes of fuel oil, ran aground on a coral reef off the Indian Ocean island on 25 July.
Despite bad weather, Prime Minister Pravind Jugnauth said 500 tonnes had been safely pumped out on Monday.
But he warned the country was preparing for the "worst-case scenario".
Mauritius is home to world-renowned coral reefs, and tourism is a crucial part of its economy.
Fuel has been transferred to shore by helicopter and to another ship owned by the same Japanese firm, Nagashiki Shipping.
France has sent a military aircraft with pollution control equipment from its nearby island of Réunion, while Japan has sent a six-member team to assist the French efforts.
The Mauritius coast guard and several police units are also at the site in the south-east of the island.
Since the weekend, volunteers have been collecting straw from fields and filling sacks to make barriers against the oil.
Others have made their own tubes with tights and hair to add to the effort, and some have been cleaning up the island's beaches.
Their actions went against an order from the government asking people to leave the clean-up to local authorities.
"People have realised that they need to take things into their hands. We are here to protect our fauna and flora," environmental activist Ashok Subron told AFP news agency on Sunday.
Mitsui OSK Lines, the operator of the ship, said on Sunday it had tried to place its own containment booms around the vessel but had not been successful owing to rough seas.
It is thought that the bulk carrier, registered in Panama, had some 4,000 tonnes of fuel aboard when it ran aground. All crew were evacuated.
More than 1,000 tonnes of oil is thought to have leaked into the waters surrounding the island nation.
Fears for the environment
Environmentalists are concerned about the impact on the country's ecosystem.
The MV Wakashio ran aground at Pointe d'Esny, a known sanctuary for rare wildlife. The area also contains wetlands designated as a site of international importance by the Ramsar convention on wetlands.
Happy Khamule from Greenpeace Africa warned that "thousands" of animal species were "at risk of drowning in a sea of pollution, with dire consequences for Mauritius' economy, food security and health".
Mauritius has "world-important populations of reptiles with unique genetic make-up", which could be among the species threatened, said Vikash Tataya, conservation director with the Mauritian Wildlife Organisation.
At a news conference, Akihiko Ono, executive vice-president of Mitsui OSK Lines "profusely" apologised for the spill and for "the great trouble we have caused".
He vowed that the company would do "everything in their power to resolve the issue".
Police in Mauritius say they have been granted a search warrant, allowing them to board the vessel take away items of interest such as the ship's log book in order to help with an investigation. The ship's captain will assist officers with their search.
On Friday, Mauritian Prime Minister Pravind Jugnauth declared a state of emergency and appealed for help.