Vatican City - Pope Francis on Friday accused both Donald Trump and Kamala Harris of being "against life" as he returned to Rome from an 12-day tour of the Asia-Pacific.

The 87-year-old pontiff's comments on the presidential hopefuls came as he defied health concerns to travel across the globe.

It was Francis' longest trip in duration and distance since becoming head of the world's nearly 1.4 billion Roman Catholics more than 11 years ago.

And despite the marathon visit, he held a long and spirited discussion with journalists onboard the papal plane returning from Singapore after his rapturous welcomes in Indonesia, Papua New Guinea, and East Timor.

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Asked about the looming US elections in November, he noted Trump's anti-immigrant policies and Harris' support for abortion rights.

"Both are against life. The one who discards migrants and the one who kills children. Both are against life," he said.

Former president Trump has promised to round up undocumented immigrants and deport them as he seeks to return to the White House in the looming November election. He also paved the way for a 2022 US Supreme Court ruling that overturned Roe v. Wade, the 1973 ruling that made abortion a national right for women – a right that Harris has pledged to restore.

"One has to choose the lesser of two evils. Who is the lesser evil? That lady or that gentleman? I don't know. Everyone have to think and make this decision according to their conscience," Francis said.

In Washington, Karine Jean-Pierre, spokesperson for President Joe Biden – himself a Roman Catholic – said that "obviously the pope speaks for himself, and I don't have any more comments."

"I have not spoken to the president about the pope's specific comments on this coming election," she said.

During his flight back to Rome, Francis also rejected media speculation by saying he would not travel to Paris in December for the reopening of Notre-Dame Cathedral, which was partially destroyed by a fire in April 2019.

He also deplored a lack of progress in negotiations to end Israel's assault on Gaza.

"Forgive me for saying so but I don't see any progress being made towards peace," he told reporters.

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The Argentine pope's epic four-nation voyage was believed by some to be foolhardy after years of health issues, from knee pain and sciatica forcing him to use a wheelchair to recent bouts of flu and bronchitis.

But the voyage clearly energized the pope – who nevertheless at times struggled to keep his eyes open during late-night liturgical readings, or to appear engaged during formal military parades.

In a lively, final inter-religious meeting in Singapore on Friday, Francis joked with young people in the audience, urging them to respect other beliefs, to avoid being "slaves" to technology, and to escape their comfort zones.

"Don't let your stomach get fat, but let your head get fat," the pope said, raising a laugh from the crowd.

"I say take risks, go out there," he said. "A young person that is afraid and does not take risks is an old person."