Pope Leo on Sunday described those who wage wars and appropriate the earth's resources as thieves who rob the world of a peaceful future, issuing a warning about the use of nuclear power on the anniversary of the Chornobyl reactor accident.

Ukraine is commemorating the 40th anniversary of the world's worst nuclear disaster on Sunday amid lingering fears that Russia's four-year-old war could spark a repeat of the tragedy.

In his weekly address after the Angelus prayer, the Pontiff said the Chornobyl accident had left a mark on humankind's collective conscience.

"It remains a warning over the use of ever more powerful technologies," the Pope, who has just returned from a 10-day tour across four African nations, said.

"I hope that at all decision-making levels, wisdom and responsibility always prevail, so that atomic power can always be used to support life and peace," he added.

Commenting on the Gospel of the day, which contained the metaphor of a sheep thief, Pope Leo said thieves came under many appearances, listing as examples "superficial lifestyles driven by consumerism", prejudices and wrong ideas.

"And let's not forget also those thieves who, by plundering the earth's resources, by fighting bloody wars or feeding evil in whichever form, are simply taking away from all of us the chance of a future of peace and serenity," he added.

Leo, the first U.S. pontiff, has attracted the ire of President Donald Trump after becoming more outspoken against war and despotism.