Countries have approved a historic deal to reverse decades of environmental destruction threatening the world's species and ecosystems at a marathon UN biodiversity summit early this morning.

The chair of the COP15 nature summit, Chinese Environment Minister Huang Runqiu, declared the deal adopted at a late-night plenary session in Montreal and struck his gavel, sparking loud applause from assembled delegates.

In doing so he overruled an objection from the Democratic Republic of Congo, which had refused to back the text, demanding greater funding for developing countries as part of the accord.

After four years of fraught negotiations, more than 190 other states rallied behind the Chinese-brokered accord aimed at saving the lands, oceans and species from pollution, degradation and the climate crisis.

The deal pledges to secure 30% of the planet as a protected zone by 2030 and to stump up $30 billion in yearly conservation aid for the developing world.

Environmentalists have compared the accord to the landmark plan to limit global warming to 1.5C under the Paris agreement, though some earlier warned that it did not go far enough.

Brian O'Donnell of the Campaign for Nature called it "the largest land and ocean conservation commitment in history".

"The international community has come together for a landmark global biodiversity agreement that provides some hope that the crisis facing nature is starting to get the attention it deserves," he said.

"Moose, sea turtles, parrots, rhinos, rare ferns and ancient trees, butterflies, rays, and dolphins are among the million species that will see a significantly improved outlook for their survival and abundance if this agreement is implemented effectively."

Marco Lambertini, head of the Worldwide Fund for Nature, said ahead of the approval sessions: "It is the equivalent to 1.5C in climate and vital to catalysing action toward a nature-positive world and holding everyone accountable.

"However, there still remain several loopholes, weak language, and timelines around actions that aren't commensurate with the scale of the nature crisis we're all witnessing, and importantly may not add up to achieve this shared global goal."