Confronted with a climate crisis threatening civilisation itself, humanity must choose between hope and surrender, UN chief Antonio Guterres told the opening plenary of a UN climate conference Monday.

"One is the path of surrender, where we have sleepwalked past the point of no return, jeopardising the health and safety of everyone on this planet," Guterres said.

"Do we really want to be remembered as the generation that buried its head in the sand, that fiddled while the planet burned?"

In a separate forum moments earlier, US Congressional leader Nancy Pelosi told the "COP25" conference that the world could still count on the United States despite President Donald Trump's decision to pull out of the Paris Agreement.

States and cities home to two-thirds of the US population are committed to the targets set by the 2015 agreement, as are all the Democratic candidates for president, according the US research groups.

"We're here to say to all of you, on behalf of the House of Representatives and the Congress of the United States, we're still in it, we're still in it," Pelosi said to applause at a forum of heads of state from climate-vulnerable nations.

Leading the 15-strong Congressional delegation, Pelosi came to Madrid even as her colleagues in the House consider articles of impeachment against Trump.

Trump has dismissed global warming as a hoax, and dismantled many of the climate and environmental protection policies set in place by his predecessor Barack Obama.

Last month Trump gave formal notice of the US withdrawal from the 196-nation Paris climate treaty, which calls for capping global warming at well below two degrees Celsius (3.6 degrees Fahrenheit), and 1.5C if possible.

- 'Forcing our country to die' -

In his impassioned appeal, Guterres cited new findings from the World Meterological Organisation (WMO) confirming that the last five years have been the hottest ever recorded.

Concentration of planet-warming CO2 in the atmosphere has also reached levels not seen in three to five million years, the WMO will report this week.

"The last time there was a comparable concentration," Guterres said, "the temperature was two to three degrees Celsius warmer, and sea levels were 10 to 20 metres (32 to 66 feet) higher than today."

A major UN science report last year reset the Paris accord's threshold for a climate-safe world from 2C to 1.5C, concluding that the global economy must be "carbon neutral" by 2050 to stay under that threshold.

"What is still lacking is political will -- to put a price on carbon, to stop subsidies on fossil fuels, to stop building coal power plants," Guterres said.

"The best available science, through the Intergovernmental Panel on Climate Change (IPCC), tells us today that going beyond that (1.5C) would lead us to catastrophic disaster."

President Hilda Heine of the Marshall Islands warned that breaching the 1.5C barrier would spell the end of her water-bound homeland.

"The most vulnerable atoll nations like my country already face death row" due to rising seas and devastating storm surges," she said via a remote video link-up.

Governments that fail to come forward with strong carbon-cutting commitments over the next year are effectively "passing sentence on our future, forcing our country to die."

- 'Leaders cannot flee' -

The talks in Madrid are focused on finalising rules for global carbon markets, and setting up a fund to help countries already reeling from climate-enhanced heatwaves, droughts, floods and storms made worse by rising seas.

Frontline negotiators describe COP25 as "technical talks" setting the stage for next year's meeting in Glasgow, where countries must confront the yawning gap between the Paris targets and current emissions.

But events outside the conference hall in Madrid may change the agenda.

"A key question will be to what extent the growing social movements throughout the world will be factored into decisions of the COP25," said Laurence Tubiana, CEO of the European Climate Foundation and, as a former negotiator for France, a main architect of the Paris Agreement.

A climate action group steeped in civil disobedience, meanwhile, laid plans to descend on the Spanish capital.

"Extinction Rebellion calls on Rebels Without Borders to come to Madrid," the group said in a tweet, using the hashtag #UltimatumCOP25.

"Extinction Rebellion reminds leaders they cannot flee the climate and ecological emergency," the group said separately in a press release.

"Civil disobedience and direct nonviolent actions coordinated by global rebels will fill Madrid's streets and squares."