"They've been charged with trespassing and interfering with business operations," said Graham Forbes, the organisation's delegation head at talks in Busan where countries have hours left to reach a landmark accord.

The four activists -- of British, German, Taiwanese and Mexican nationality -- boarded the boat off South Korea on Saturday in an action that Greenpeace said lasted 12 hours.

Nearly 200 nations are in Busan to negotiate, but some delegations are now advocating for an extension of the talks, accusing a handful of nations of obstructing an ambitious agreement.

"We hope that the courage shown by these activists, who endured freezing overnight temperatures in order to state loud and clear that this treaty fails unless it cuts plastic production will act as an inspiration to delegates," Forbes said.

"We urge the South Korean authorities to release the activists immediately," he added.

On Saturday Greenpeace said the tanker Buena Alba, anchored off the Hanwha TotalEnergies complex, was scheduled to pick up propylene, which is used to manufacture plastic.

The environmental group said its activists had boarded the vessel peacefully and painted "PLASTIC KILLS" on the side of the boat before setting up a camp.

A spokesman for Wooil Shipping, the Korean company managing the vessel, told AFP on Saturday that "we haven't been able to load any cargo all day".

Whether to cut new plastic production has been a key sticking point in the week of negotiations in Busan.

Dozens of countries, backed by environmental groups, insist a treaty without production cuts will fail to solve the problem, but a group of largely oil-producing states is fiercely opposed.

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