World leaders must choose between saving the world or condemning "humanity to a hellish future", UN chief Antonio Guterres warned Thursday at a preparatory meeting for a key climate summit next month.

The COP26 UN climate summit, which begins on 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.

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.

"It is essential for all humanity that we fulfil the promise of the Paris Agreement. I cannot emphasise enough that time is running out. Irreversible climate tipping points lie alarmingly close," Guterres told ministers from several countries attending the preparatory meeting in Milan, which runs till Saturday.

"We can either save our world or condemn humanity to a hellish future."

The UN secretary general said "the single most effective step we can take to limit temperature rise is phasing out coal, beginning with no new coal power plants".

Welcoming a recent announcement by China on ending international financing of coal power, he asked private finance "to quickly follow suit and stop financing coal".

Guterres had earlier also asked youths to maintain pressure on global leaders to do more to fight climate change.