Ministers tasked with securing a historic climate-saving pact in Paris sought Tuesday to resolve the most volatile issues, such as mustering hundreds of billions of dollars to help the developing world.

The 195-nation UN talks have been billed as the last chance to avert the worst consequences of global warming: deadly drought, floods, storms, and rising seas that will engulf islands and densely populated coastlines.

To reach an elusive deal by the ­organizers' Friday deadline, however, the ministers must first resolve a handful of decades-old disputes that have blocked the path to the first truly universal climate pact.

Nations remain divided over how to provide developing nations with financing to cope with global warming.

They have also clashed on how far to limit planetary overheating, how to share the burden between rich and poor nations, and how to review progress in greenhouse gas reduction.

"They are finally doing the dirty work of negotiating, which is very hard," said Jennifer Morgan, an observer at the talks and a global climate analyst at the ­Washington-based World Resources Institute.

"You are finally starting to see the really hard bargaining and arguing that has to happen. It is a good thing, because otherwise they would still be standing their positions."

Observers said a new confidence was emerging in Paris, a hopeful sign six years after the spectacular failure of the last attempt to reach a global deal in Copenhagen, where talks were ­fractured by distrust between rich and poor ­countries.

"Those of us who have been watching these for a long time think there has been a spirit of cooperation," Greenpeace political advisor Ruth Davis told journalists.

"But cooperation has to manifest itself in something real and meaningful," said Davis, who described the complex negotiations as being akin to a 12-side Rubik's cube.

The Paris accord - which will take effect in 2020 - will seek to limit emissions of heat-trapping greenhouse gases driven by the burning of coal, oil and gas, the current backbone of the world's energy supply.

The goal is to limit global warming to less than 2 C above pre-Industrial ­Revolution levels.

Negotiations are making headway on every front, said France's top negotiator, Laurence Tubiana. "We are advancing on everything," she said. "There is no issue where we are blocked. None."

But one potential deal-buster is money. In 2009, rich countries promised to muster $100 billion per year beginning in 2020 to help developing nations make the costly shift to clean energy and cope with the impacts of global warming.

But it remains unclear how the pledged funds will be raised, and developing countries are pushing for a promise that the amount will be ramped up beyond 2020.