The world's major economies must "stretch to do more" at next month's United Nations climate talks to show they are serious about wanting to tackle global warming, US Climate Envoy John Kerry has said.

The COP26 conference in Glasgow aims to secure more ambitious climate action from the nearly 200 countries that signed the 2015 Paris Agreement to limit global warming to well below 2 degrees Celsius - and to 1.5 degrees preferably - above pre-industrial levels.

"We now have about 55% of global GDP committed to undertake tracks that will hold the temperature to 1.5 degrees. There are other countries now sharpening their pencils," Mr Kerry said during a pre-COP26 meeting in Milan, Italy.

"Well below 2 degrees Celsius means well below ... the common sense meaning of that is not 1.9, 1.8 or 1.7 (degrees)," he added.

New energy and funding pledges from the US and China have raised negotiators' hopes, but many G20 countries, including major polluters such as China and India, have yet to announce updates of their short-term climate action plans.

Greta Thunberg accused governments of 'shamelessly congratulating themselves' for insufficient pledges to cut emissions

Youth climate activists including Sweden's Greta Thunberg, who was in Milan this week, have demanded that policymakers match words with action and stump up billions of dollars to wean the world off fossil fuels.

They have also called for a transparent climate finance system and more grants to help the people most exposed to the impact of climate change.

Wealthy nations that pledged a decade ago to mobilise $100bn (€86.2bn) a year to help vulnerable countries adapt and transition to cleaner energy are still short of their 2020 goal.

The world's major economies must show they are serious about wanting to tackle global warming, US Climate Envoy John Kerry has said

Mr Kerry said he expected donors to fulfil the pledge, but added that a post-2025 finance plan "with an emphasis not just on billions, but on trillions" would be required.

"(The) private sector is needed for this ... We'll be announcing one specific agenda item in conjunction with the World Economic Forum," he said, without giving further details.

EU climate commissioner Frans Timmermans echoed Mr Kerry's call for radical and swift action.

"We're fighting for the survival of humanity," he told reporters.

Asked about coal mining, Mr Timmermans said the industry would gradually disappear even without specific climate action because it would eventually become economically unviable.

"I'd be highly surprised if there is still a significant coal mining industry after 2040," he said.

China and India, the world's top two coal producers, still rely on coal-fired power stations for a big share of their electricity supply.

"We're in a very constructive dialogue with India and China ... there is a wish by both countries to be part of the success," Mr Timmermans added.