More than 75 leaders are expected in Baku over two days but the heads of some of the most powerful and polluting economies are not attending this year's summit.

Just a handful of leaders from the G20 -- which accounts for nearly 80 percent of planet-heating greenhouse gas emissions -- are expected in Baku, including UK Prime Minister Keir Starmer.

The British leader will unveil an "ambitious" update to the UK's climate goals later today, and said he wanted his country "to show leadership on the climate challenge."

Joe Biden, Xi Jinping, Narendra Modi and Emmanuel Macron are among G20 leaders missing the event, where uncertainty over future US unity on climate action hung over the opening day.

"It's not an ideal situation," acknowledged Steven Guilbeault, Canada's environment minister.

"But in 30 years of COP, it's not the first time that we've faced obstacles," he told AFP. "Certainly, everything is still possible."

Washington's top climate envoy John Podesta is seeking to reassure countries in Baku that Trump's re-election will not end US efforts on global warming, even if the issue will be "on the back burner".

But despite calls for global cooperation, the opening day got off to a rocky start, with feuds over the official agenda delaying by hours the start of formal proceedings in the stadium venue near the Caspian Sea.

"This will be a tough COP," said Fernanda Carvalho, global climate and energy policy lead at WWF.

"Countries are divided. There is a lack of trust," she told AFP, and divisions over climate finance "will be reflected in every room of those negotiations."

- 'Climate debt' -

The top priority at COP29 is landing a hard-fought deal to boost funding for climate action in developing countries.

These nations -- from low-lying islands to fractured states at war -- are least responsible for climate change but most at risk from rising seas, extreme weather and economic shocks.

Some are pushing for the existing pledge of $100 billion a year to be raised ten-fold at COP29 to cover the future cost of their nations shifting to clean energy and adapting to climate shocks.

Babayev, a former oil executive, told negotiators that trillions may be needed, but a figure in the hundreds of billions was more "realistic".

That has angered developing countries and NGOs, who argue rich countries and historical emitters owe a "climate debt."

Nations have haggled over this for years, with disagreements over how much should be paid, and who should pay it, making meaningful progress next to impossible ahead of COP29.

Developing countries warn that without adequate finance, they will struggle to offer ambitious updates to their climate goals, which countries are required to submit by early next year.

The small group of developed countries that currently contributes the money wants the donor pool expanded to include other rich nations and top emitters, including China and the Gulf states, something firmly rejected by Beijing.

UN climate chief Simon Stiell has tried to convince rich countries that climate finance is not charity, but a downpayment on a safer, wealthier planet.

"The climate crisis is fast becoming an economy killer," he warned.

"Climate action is global inflation insurance," he added.

He also sought to reassure the talks that recent "political events" would not derail global climate diplomacy.

"Our process is strong. It's robust, and it will endure."

Around 50,000 people are attending summit in Azerbaijan, a petrostate wedged between Russia and Iran, including the leaders of many African, Asian and Latin American countries beset by climate disasters.