UN chief Antonio Guterres on Saturday urged world leaders to declare a "state of climate emergency" and shape greener growth after the coronavirus pandemic, as he opened a summit marking five years since the landmark Paris Agreement.

The Climate Ambition Summit, being held online, comes as the United Nations warns current commitments to tackle rises in global temperatures are inadequate.

The commitments made in Paris in 2015 were "far from enough" to limit temperature rises to 1.5 degrees Celsius, the UN secretary-general said in his opening address to the summit, which is co-hosted by Britain and France.

"If we don't change course, we may be headed for a catastrophic temperature rise of more than 3.0 degrees this century," he said.

"That is why today, I call on all leaders worldwide to declare a State of Climate Emergency in their countries until carbon neutrality is reached," he added, arguing the recovery from COVID-19 presented a rare opportunity to recalibrate growth.

British Prime Minister Boris Johnson told the climate summit that a traumatic year of pandemic was ending with the hope of vaccines coming on-stream.

"My message to you all is that together, we can use scientific advances to protect our planet, our biosphere against a challenge far worse, far more destructive, than even the coronavirus pandemic," he said.

For its part, Britain was acting on climate not because it was a nation of "mung bean-eating eco freaks" but because scientific progress would allow the creation of "millions" of green jobs, Johnson added.

Speaking slots were handed to countries that submitted the most ambitious plans to accelerate their Paris promises.

These include Honduras and Guatemala, which were hit last month by a pair of monster hurricanes, as well as India, which is battling increasingly erratic weather patterns and air pollution.

Business figures set to speak include Tim Cook, the chief executive of Apple, which has committed to making its whole supply chain carbon neutral by 2030.

But major economies including Australia, Brazil and South Africa are absent.

Brazil's far-right President Jair Bolsonaro has dismissed the scale of destruction to the Amazon rainforest opened up by his climate-sceptic policies.

The United States, the world's second-largest polluter after China, left the Paris Agreement under President Donald Trump. President-elect Joe Biden plans immediately to re-enter the accord, and has set a goal of carbon neutrality by 2050.

"We haven't come close to the bold action that's needed, and today, we have no time to waste," Biden said in a statement Saturday, reiterating a pledge to convene leaders of major economies for his own climate summit within 100 days of taking office next month.

 'Moment of accountability'

Speakers at Saturday's summit were delivering short video messages, with organisers saying they would announce "new and ambitious climate change commitments" and that there would be "no space for general statements".

Under the Paris climate accord, signatories committed to action to limit temperature rises to "well below" 2.0 Celsius above pre-industrial levels, and to try to restrain them to 1.5C.

But the UN warned this week that temperatures remain on course to rise more than 3.0C this century, creating a crisis that will "dwarf the impacts of Covid-19".

The summit is a stocktaking exercise five years after the Paris accord was signed and a prelude to the UN's COP26 climate change conference in Scotland next November. That was meant to happen last month but was delayed by the pandemic.

Greenpeace called Saturday a "moment of accountability for leaders".

Under the Paris deal's "ratchet" mechanism, countries are required to submit renewed emissions cutting plans - termed Nationally Determined Contributions or NDCs - every five years.

The deadline for this is December 31.

Countries are set to announce efforts to reduce national emissions, long-term strategies and financial commitments to support the most vulnerable.

More than 110 countries have committed to becoming carbon neutral by 2050. China, the world's biggest polluter, announced in September plans to achieve net-zero emissions by 2060.

The summit comes as EU leaders on Friday committed to the goal of reducing emissions by 55 percent by 2030 compared to 1990 levels.

Britain -- out of the EU since January -- this month announced it would seek to reduce emissions by 68 percent over the same period.

Johnson has presented plans for a "green industrial revolution" creating up to 250,000 jobs. Before the summit opened, he committed to ending all direct British government support for the fossil fuel sector overseas.