UN chief Antonio Guterres urged young people on Thursday to keep up their pressure on world leaders, praising them for "setting a powerful example" ahead of a vital climate summit next month.

"The window of opportunity to prevent the worst impacts of the climate crisis is closing quickly," he told hundreds of youth activists gathered in Milan.

"We know what needs to be done and we have the tools to do it. Young people have been in the forefront of putting forward positive solutions, advocating for climate justice and holding leaders to account."

The COP26 UN climate summit, which begins October 31 in Glasgow, is being billed as crucial to the continued success of the 2015 Paris Agreement.

The landmark 2015 accord commits nations to limiting global temperature rises to "well below" 2 degrees Celsius above pre-industrial levels through sweeping emissions cuts.

Countries also promised to work towards a safer temperature cap of 1.5C.

But emissions have continued to rise in the six years since the deal, while the latest round of national carbon-cutting pledges puts Earth on track to warm a "catastrophic" 2.7C this century, according to a UN assessment this month.

Guterres urged delegates in Glasgow to bring their emissions plans in line with a 1.5C pathway.

"This means that they must commit to net-zero by mid-century, with ambitious 2030 targets, and clear plans to achieve them," he said.

The activists in Milan will outline their priorities for climate action in a joint communique to be presented to ministers meeting Saturday as part of pre-COP preparations
The activists in Milan will outline their priorities for climate action in a joint communique to be presented to ministers meeting Saturday as part of pre-COP preparations AFP / MIGUEL MEDINA

The UN chief also called on developed nations at COP26 to make good on their promise to deliver $100 billion each year to countries already bearing the brunt of climate disasters.

The activists in Milan will outline their priorities for climate action in a joint communique to be presented to ministers meeting Saturday as part of pre-COP preparations.

"Your solidarity and demands for action set a powerful example," Guterres told the youth delegates.

"We need national leaders to follow your example and ensure the ambition and results we need at COP26 and beyond."

Britain's Prime Minister Boris Johnson, whose country will host COP26, said via video link that "young people around the world are already paying the price for the reckless actions of their elders."

"There is still just enough time to put on the brakes. We know what needs to be done, we just have to get on with it. Change on the scale we need is perfectly possible."

Johnson summed up the summit aims as: "Coal, cars, cash and trees."

"We want to move away from using coal as a way we generate electricity and move towards renewables," he said.

"We want everybody to stop using internal combustion engine vehicles. We want to raise the funds that the whole world needs -- the developing world in particular -- to tackle climate change, and we need to plant hundreds of billions if not trillions of trees," Johnson said.

"If we do that, we can make COP a success."