"We urge you to stand up for the people of the Global South and we insist: no deal in Baku is better than a bad deal, and this is a very, very bad deal because of the intransigence of developed countries," said the letter addressed to the G77 bloc of developing nations and China.

The talks in Baku, which were scheduled to conclude after 12 days on Friday, extended through the night into Saturday as poor nations rejected a draft proposal in which the developed world would provide $250 billion a year to countries worst hit by climate change.

The proposal notes a target of $1.3 trillion per year to cope with rising temperatures and disasters, but most would come from private sources as wealthy nations say it is not politically realistic to tap further into government coffers.

In an accompanying letter, the non-governmental organisations accused the United States, European Union, Britain and other developed nations of seeking to use the deal as proposed to make a "complete exit from any legal obligations to provide climate finance for developing countries".

"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," they wrote.

Signatories included representatives of ActionAid, Amnesty International, CAN International, Christian Aid and 350.org.

The talks at a stadium in the Azerbaijani capital have focused on finding a new climate finance goal, with the $100 billion a year provided by wealthy nations under a previous commitment set to expire.

The talks are taking place on what is poised to be the hottest year on record, with rising droughts, fires and storms taking a deadly toll, but also against the backdrop of a political resurgence of right-wing critics of the green agenda, including the victory of Donald Trump in the US presidential election.