The United Nations has sounded the alarm over an oceans "emergency" and leaders gathering in Nice will be called to commit money and stronger protections for the seas.

The UN Ocean Conference, starting Monday, seeks to turn a corner as nations feud over deep-sea mining, plastic litter and exploitative fishing, against a backdrop of wider geopolitical tensions.

"We have a duty to mobilise, because the science is clear and the facts are there," said French President Emmanuel Macron in Monaco on Sunday, where he attended a pre-conference event with Britain's Prince William, among other high-level dignitaries.

Some 60 heads of state and government are expected in Nice, including Brazilian President Luiz Inacio Lula da Silva and his Argentine counterpart Javier Milei.

"The planet can no longer tolerate broken promises," said Lula on Sunday.

"Either we act, or the planet is in danger."

- 'No excuses' -

On Sunday, Macron travelled by boat from Monaco to Nice and toured a conference pavilion made to look like the cavernous belly of a whale.

In the evening, he was scheduled to host leaders for a dinner of Mediterranean fish.

France has deployed 5,000 police to Nice for the five-day summit where scientists, business leaders and environmental activists were also to attend in big numbers.

A strong turnout was expected from Pacific Island nations, whose delegations will urge financial aid to combat the rising seas, marine trash and plunder of fish stocks.

The United States under President Donald Trump -- whose recent push to fast-track seabed mining in international waters sparked global outrage -- was not expected to send a delegation.

Conservationists have warned the summit -- which will not produce a legally binding agreement -- risks being a mere talking shop unless leaders come up with concrete proposals to restore marine health.

The UK announced on Sunday that it intended to impose a partial ban on bottom trawling, a destructive fishing method that involves dragging huge nets across the ocean floor.

In a statement, Greenpeace welcomed the decision but said it was "long overdue".

On Saturday, Macron said France would restrict bottom trawling in some of its marine protected areas but was criticised for not going far enough.

- Pay up -

Nations will also face calls to cough up the missing funds to protect 30 percent of the world's oceans by 2030, a target agreed by nearly 200 countries in 2022.

"We've created this sort of myth that governments don't have money for ocean conservation," Brian O'Donnell, director of Campaign for Nature, told reporters.

"There is money. There is not political will," he said.

So far, only around eight percent of oceans are designated marine conservation zones and even less are considered truly protected.

Greenpeace says at this rate, it could take another 82 years to reach the 30 percent goal.

France's environment minister Agnes Pannier-Runacher told reporters on Sunday she expected "important announcements" in Nice that would bring a greater share of the world's oceans under conservation protection.

Earlier this week, Samoa declared 30 percent of its national waters under protection with the creation of nine new marine parks.

Conservationists hope others at the summit follow suit.

"All eyes should be on the many Pacific leaders attending... Their ambition and dedication to ocean protection can serve as inspiration to all countries," said Kevin Chand from the nonprofit group Pristine Seas.

Another summit priority will be inching towards the numbers required to ratify a global treaty on harmful fishing subsidies, and another on protecting the high seas beyond national control.

France is also spearheading a push in Nice to build support for a moratorium on deep-sea mining ahead of a closely-watched meeting of the International Seabed Authority in July.