In a joint statement issued as they held a summit in Rio, the leaders spoke only of the trillions of dollars needed coming "from all sources," without specifying.

They also spoke of phasing out "inefficient fossil fuel subsidies" -- instead of fossil fuels themselves, which play a key role in global warming.

"The leaders are kicking the can back to Baku," the capital of Azerbaijan where UN climate talks are happening, said Mick Sheldrick, co-founder of the advocacy group Global Citizen.

"They haven't stepped up to the challenge... This is probably going to make it harder to achieve an agreement," he said.

UN chief Antonio Guterres said ahead of attending the G20 summit that its members needed to show "leadership and compromise" to rescue the deadlocked climate talks.

Negotiators in Azerbaijan are unable to reach agreement on a deal for climate investments in developing nations after more than a week of deliberations.

The talks are stuck over the final figure, the type of financing, and who should pay, with Western countries wanting China and wealthy Gulf states -- many reliant on fossil fuel revenues -- to join the list of donors.