Pope Francis called for humanity to unify in the face of climate disaster "before it's too late" during an address on the sidelines of the United Nations.

The pontiff also declared the refugee crisis one of the greatest challenges of our time and warned the "Third World War" was already being fought in the spate of conflicts around the globe.

"It's time to work together to stop the ecological catastrophe before it's too late," said the 86-year-old during an onstage video conference with former US president Bill Clinton, in the opening of this year's Clinton Global Initiative.

He said he was writing a new papal document as a follow-up to his landmark 2015 "encyclical" that repositioned the Roman Catholic Church on environmentalism.

The pope's comments come as climate monitors predict 2023 will be the hottest in human history, with this summer marked by heatwaves, droughts and wildfires.

Pope Francis has made the climate crisis a key plank of his decade-long leadership, though it is unclear how much it has impacted politically conservative followers of the Church.

Turning to migration, he said it was important to remember "we are not talking about numbers, but about people: men, women and children.

"When we talk about migration, let's think about the eyes of the children we've seen in refugee camps."

On the world's conflicts, the pope said World War III was "being fought piecemeal," adding it was time to "return to dialogue and diplomacy."

"Let the designs of conquest and military aggression cease," he said.

He also reflected on the work of the Bambino Gesu Children's Hospital in Italy, which he said has treated more than 2,000 young patients from Ukraine who had escaped from the country without their parents or relatives -- adding this could serve as a global model.

World leaders are due to meet Wednesday at a Climate Ambition Summit, with G20 countries under pressure from United Nations Secretary-General Antonio Guterres to accelerate their actions in the hope of keeping the 2015 Paris agreement alive.