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

The 87-year-old pontiff's comments on the US presidential hopefuls came onboard the papal plane returning from Singapore after his rapturous welcomes in Indonesia, Papua New Guinea and East Timor.

Asked about the looming US elections in November, he noted Trump's anti-immigrant policies and Harris's 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 illegal 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, spokeswoman 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 the war in Gaza.

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