Growing anxiety over Trump's unilateral push to fast-track deep-sea mining in international waters shot to the surface at the opening of the UN Ocean Conference in southern France.

"I think it's madness to launch predatory economic action that will disrupt the deep seabed, disrupt biodiversity, destroy it and release irrecoverable carbon sinks -- when we know nothing about it," said French President Emmanuel Macron.

Imposing a moratorium on seabed mining was "an international necessity", said Macron.

The number of countries opposed to seabed mining rose to 36 on Monday, according to a tally kept by the Deep Sea Conservation Coalition, an umbrella group of non-governmental organisations.

Trump was not among the roughly 60 heads of state and government in the seaside town of Nice but his spectre loomed large as leaders defended the global multilateralism he has spurned.

Of particular concern, his move to sidestep the International Seabed Authority (ISA) and issue permits directly to companies wanting to extract nickel and other metals from waters beyond US jurisdiction.

Brazilian President Luiz Inacio Lula da Silva called for "clear action" from the seabed authority to end a "predatory race" for deep-ocean minerals.

"We now see the threat of unilateralism looming over the ocean. We cannot allow what happened to international trade to happen to the sea," he said.

The deep sea, Greenland and Antarctica were "not for sale", Macron said in further remarks directed clearly at Trump's expansionist claims.

The ISA, which has jurisdiction over the ocean floor outside national waters, is meeting in July to discuss a global mining code to regulate mining in the ocean depths.

UN Secretary-General Antonio Guterres said he supported these negotiations and urged caution as countries navigate these "new waters on seabed mining".

"The deep sea cannot become the wild west," he said, to applause from the plenary floor.

- 'Wave of hope' -

Island nations also spoke out against seabed mining, which scientists warn could result in untold damage to ecosystems largely unexplored by humanity.

"Here in Nice, we can feel that the looming threat of deep sea mining, and the recent reckless behaviour of the industry is seen by many states as unacceptable," said Megan Randles from Greenpeace.

Meanwhile, a flurry of last-minute signatures in Nice brought a treaty to protect the 60 percent of the world's oceans outside national jurisdiction closer to law.

Macron told reporters that 55 nations had ratified the high seas treaty, just five shy of the number required for its enactment.

The president gave two thumbs up and a broad grin as he posed with the new signatories, and said the accord would come into force by January 1, 2026.

"Today's surge of ratifications for the High Seas Treaty is a tidal wave of hope and a huge cause for celebration," Rebecca Hubbard, director of the High Seas Alliance, told AFP.

- 'Prove you're serious' -

On Monday, the United Kingdom announced plans to extend a partial ban on bottom trawling in some of its protected marine areas, following a similar move by France at the weekend.

Greece, Brazil and Spain took the opportunity Monday to announce the creation of new marine parks, following recent similar action from Samoa.

And French Polynesia unveiled what will be the world's largest marine protected zone, said the International Union for Conservation of Nature.

Just eight percent of global oceans are designated for marine conservation, despite a globally agreed target to achieve 30 percent coverage by 2030.

Macron said he hoped that coverage would grow to 12 percent by the summit's close on Friday.

Environment groups say that for marine parks to be considered truly protected they need to ban trawling and other harmful activities, and be properly funded.

Wealthy nations face pressure in Nice to commit money to make that level of ocean conservation a reality.

Small island states are leading the charge for money and political support to better combat rising seas, marine trash and the plunder of fish stocks that hurt their economies.

"We say to you, if you are serious about protecting the ocean, prove it," said President Surangel Whipps Jr of Palau, a low-lying Pacific nation.

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