UN talks aimed at saving mankind from the dire impacts of global warming enter their crunch phase Monday with ministers beginning a frenetic week of negotiations to seal a historic 195-nation agreement in Paris.

The envisaged accord seeks to revolutionise the world's energy industry by replacing coal, oil and gas with renewable sources that do not emit heat-trapping greenhouse gases.

The conference in the French capital crowns more than two decades of obstacle-strewn negotiations to curb climate change, which threatens to make Earth increasingly hostile to human existence.

Speaking before thousands of worshippers in the Vatican Sunday, Francis said he prayed that leaders would muster "the courage to keep as their guiding criterion the well-being of the entire human family".

The talks opened November 30 with a record-breaking gathering of 150 world leaders who issued a chorus of warnings about mankind's fate if planet warming went unchecked.

"The future is one that we have the power to change right here, right now, but only if we rise to this moment," US President Barack Obama told the summit.

Negotiators spent the rest of the week trying to address the many deep and complex divisions among countries with competing national interests -- rows that have condemned previous UN efforts to failure.

- Cautious hopes -

While none of the major arguments was resolved, negotiators did meet a Saturday deadline to produce a draft blueprint for the accord, showing enough compromise to offer hopes for cautious optimism.

Environment and foreign ministers take the blueprint up on Monday, seeking to eliminate hundreds of bracketed words or sentences that denote disagreement.

Small island nations most vulnerable to rising sea levels and stronger storms -- so often railroaded by the powerful in the UN talks -- said they were relieved their voices were being heard in Paris.

"We would have wished to be further along than we are at this point, but the text being forwarded so far reflects our key priorities," said Thoriq Ibrahim from the Maldives and chair of the Alliance of Small Island States.

China, after being accused of contributing to the spectacular failure of the last effort to forge a global climate pact in Copenhagen six years ago, offered encouraging words.

"We are very happy to have this progress. The political will is there from all parties," Chinese climate envoy Su Wei told reporters.

Still, all those directly involved, as well as environmental groups who follow the negotiations closely, emphasised success was not even close to being assured.

"Let's be frank: all the difficult political issues remain unresolved," European Climate and Energy Commissioner Miguel Arias Canete said on Saturday.

"Next week is the week of compromise."

- Achieving the impossible -

France has set a December 11 deadline for ending the talks, giving a seemingly-impossibly short period of time to settle enduring rows that primarily pit rich nations against developing ones.

South African negotiator Nozipho Mxakato-Diseko drew on her nation's late peace icon in a bid to inspire others.

"In the words of Nelson Mandela, it always seems impossible until it is done," said Mxakato-Diseko, who is also the head of a powerful 134-nation bloc of developing countries known as the "G77 plus China".

Scientists warn Earth will become increasingly dangerous for mankind as it warms, with rising sea levels consuming islands and populated coastal areas, as well as more catastrophic storms and severe droughts.

The planned accord would seek to cap warming at 2.0 degrees Celsius (3.6 degrees Fahrenheit) below pre-Industrial Revolution levels -- or lower.

Many nations, especially smaller, poorer ones, want a target of 1.5 degrees C.

The United States, China, India and some of the other biggest polluting nations want to enshrine the 2 C goal, which would allow them to continue emitting gases for longer.

Among the biggest disputes is a demand by developing nations for hundreds of billions of dollars to pay for the costly shift to renewable energy technologies, and cope with the impacts of climate change.

Rich nations committed six years ago to begin channelling $100 billion (92 billion euros) a year annually to developing nations from 2020, when the Paris agreement would enter into force.

But United States and other wealthy countries have yet to show how this would happen, and developing countries insist the Paris agreement must enshrine escalating amounts of money from 2020.

There is also still a big divide over how to review pledges by nations to cut their emissions, and when architecture should be put in place for five-yearly reviews that could pressure nations to increase their ambitions.