Pope Leo on Monday urged "concrete actions" on climate change and complained that some leaders lacked the will to act, as he addressed religious dignitaries on the sidelines of the COP30 summit.

The Vatican released the American pope's address to churches of the southern hemisphere assembled on the sidelines of the UN climate talks in Belem, Brazil, in which he called the Amazon region "a living symbol of creation with an urgent need for care".

"Creation is crying out in floods, droughts, storms and relentless heat," the pope said.

"One in three people live in great vulnerability because of these climate changes. To them, climate change is not a distant threat, and to ignore these people is to deny our shared humanity," he added.

"What is failing is the political will of some."

The UN climate negotiations enter their final stretch this week, with nations split on key issues as government ministers began arriving on Monday to take over negotiations.

"There is still time to keep the rise in global temperature below 1.5C, but the window is closing," warned Leo, who called for "concrete actions", while championing the landmark Paris Agreement.

The historic 2015 accord, from which US President Donald Trump has said he will withdraw the United States for the second time, aims to keep temperature rises "well below" 2C compared to pre-industrial levels and, if possible, to 1.5C.

The Paris Agreement was the "strongest tool for protecting people and the planet", Leo said, decrying a lack of effort by some leaders, whom he did not name.

"True leadership means service and support on a scale that will truly make a difference," he said, urging firmer climate action to bring about "stronger and fairer economic systems".

"Let us send a clear global signal together: nations standing in unwavering solidarity behind the Paris Agreement and climate cooperation," he said.

Since being made pope in May, the Chicago-born pontiff – who spent about 20 years as a missionary in Peru – has urged more pressure on governments to stop climate change. (AFP)