Wealthy nations raised their offer for climate finance for poorer nations to $300 billion Saturday at COP29 negotiations, with growing doubts that all countries will leave happy with the outcome.

Negotiators worked through the night in a sports stadium in the Caspian Sea city of Baku in a search for compromise as the two-week COP29 conference dragged into an extra day.

In a year set to be the hottest ever recorded, developing nations bearing the brunt of rising drought and disasters flatly rejected Friday an initial offer of $250 billion per year by 2035.

On Saturday, wealthy countries — whose ranks include the European Union, the United States, Britain and Japan — raised the sum to $300 billion, said multiple sources with close knowledge of the negotiations.

The revised offer from rich countries came with conditions in other parts of the broader climate deal under discussion in Azerbaijan.

The EU in particular wants an annual review of global efforts to phase out fossil fuels, which are the main drivers of global warming.

This has run into opposition from Saudi Arabia, which has sought to water down a landmark pledge to transition away from oil, gas and coal made at COP28 last year.

"We will not allow the most vulnerable, especially the small island states, to be ripped off by the new, few rich fossil fuel emitters," said German Foreign Minister Annalena Baerbock.

Irish climate minister Eamon Ryan said he was "hopeful" for a deal but that a clearer picture would emerge later in the day, when a new text is expected.

Ryan told AFP developing countries needed money "but also we have to put a halt to the advance of fossil fuels."

A coalition of more than 300 activist groups accused historic polluters most responsible for climate change of skirting their obligation, and urged developing nations to stand firm.

"You claim to champion a rules-based system, yet flout the rules when they don't suit your interests, putting at risk billions of people and life on Earth," the non-governmental organisations wrote in an open letter.

Wealthy nations counter that it is politically unrealistic to expect more in direct government funding.

The US earlier this month elected former president Donald Trump, a sceptic of both climate change and foreign assistance, and a number of other Western countries have seen right-wing backlashes against the green agenda.

The draft deal posits a larger overall target of $1.3 trillion per year to cope with rising temperatures and disasters, but most would come from private sources.

Even $250 billion would be a step up from the $100 billion now provided by wealthy nations under a commitment set to expire.

A group of developing countries had demanded at least $500 billion, with some saying that increases were less than met the eye due to inflation.

Experts commissioned by the United Nations to assess the needs of developing countries said $250 billion was "too low" and by 2035 rich nations should be providing at least $390 billion.

This figure was taken up by Brazil, the host of next year's COP30, which says $390 billion should be the sole responsibility of wealthier countries.

The US and EU have wanted newly wealthy emerging economies like China — the world's largest emitter — to chip in.

China, which remains classified as a developing nation under the UN framework, provides climate assistance but wants to keep doing so on its own voluntary terms.

While China has generally taken a low-key and cooperative stance in Baku, oil-rich Saudi Arabia has pushed hard for weaker language on fossil fuels and, like China, has fought against being obliged to provide aid, one veteran activist from a developing country said.

Azerbaijan, an authoritarian state that relies on oil and gas exports, has been accused of lacking the experience and bandwidth to steer such complex negotiations.

Its leader Ilham Aliyev opened the conference by railing against Western nations and hailing fossil fuels as a "gift of God".