Pope Francis on Friday slammed both US presidential candidates for what he called anti-life policies on abortion and migration, and he advised American Catholics to choose who they think is the “lesser evil” in the upcoming US elections.

“Both are against life, be it the one who kicks out migrants, or be it the one who kills babies," Francis said.

The Argentine Jesuit was asked to provide counsel to American Catholic voters during an airborne news conference while he flew back to Rome from his four-nation tour through Asia. Francis stressed that he is not an American and would not be voting.

Neither Republican candidate Donald Trump nor the Democratic candidate, Kamala Harris, was mentioned by name.

But Francis nevertheless expressed himself in stark terms when asked to weigh in on their positions on two hot-button issues in the US election — abortion and migration — that are also of major concern to the Catholic Church.

Francis has made the plight of migrants a priority of his pontificate and speaks out emphatically and frequently about it. While strongly upholding church teaching forbidding abortion, Francis has not emphasised church doctrine as much as his predecessors.

Francis said migration is a right described in Scripture and that anyone who does not follow the Biblical call to welcome the stranger is committing a “grave sin”.

He was also blunt in speaking about abortion. “To have an abortion is to kill a human being. You may like the word or not, but it's killing,” he said. “We have to see this clearly.”

Asked what voters should do at the polls, Francis recalled the civic duty to vote.

“One should vote, and choose the lesser evil,” he said. “Who is the lesser evil, the woman or man? I don't know.

“Everyone in their conscience should think and do it,” he said.

It's not the first time Francis has weighed in on a US election. In the run-up to the 2016 election, Francis was asked about Trump's plan to build a wall at the US-Mexico border. Francis declared then that anyone who builds a wall to keep out migrants “is not Christian.”

In responding Friday, Francis recalled that he celebrated Mass at the US-Mexico border and “there were so many shoes of the migrants who ended up badly there”.

Trump pledges massive deportations, just as he did in his first White House bid, when there was a vast gulf between his ambitions and the legal, financial and political realities of such an undertaking.

The US bishops conference, for its part, has called abortion the “preeminent priority” for American Catholics in its published voter advice. Harris has strongly defended abortion rights and has emphasised support for reinstating a federal right to abortion.

In his comments, the pope added: “On abortion, science says that a month from conception, all the organs of a human being are already there, all of them. Performing an abortion is killing a human being. Whether you like the word or not, this is killing. You can't say the church is closed because it does not allow abortion. The church does not allow abortion because it's killing. It is murder.”

However, cells are only beginning the process of developing organs in the earliest weeks of pregnancy. The American College of Obstetricians and Gynecologists says that by 13 weeks, all major organs have formed. For example, cardiac tissue starts to form in the first two months — initially a tube that only later evolves into the four chambers that define a heart.