Pope Francis has criticized former US President Donald Trump for his plan to deport millions of immigrants and US Vice President Kamala Harris for supporting abortion rights.

When asked about the US presidential election on a flight from Singapore to Rome, Pope Francis said refusing immigrants was a "grave" sin and likened abortion to "murder". He said American Catholics will have to "choose the lesser evil" in the November election.

Francis was speaking to reporters after a grueling 12-day tour of Southeast Asia and Oceania. Although the pope did not mention Trump and Harris by name, he did refer specifically to their politics. Despite criticizing both candidates, he said Catholics should vote in the election.

"You have to choose the lesser evil. Who is the lesser evil? That lady or that gentleman? I don't know. Everyone with a conscience (must) think and do it," said 87-year-old Pope Francis.

American Catholics, about 52 million of them nationwide, are often seen as key undecided voters. In some states, including Pennsylvania and Wisconsin, more than 20 percent of adults are Catholic.

Francis, head of some 1.4 billion Catholics worldwide, is usually cautious about national political elections. But he often strongly criticizes abortion, which is forbidden according to Catholic teachings. He has previously criticized Trump's anti-immigrant rhetoric. In 2016, the Pope said that Trump was "not Christian" in his views. Today he said that the policies of both candidates are "against life".

"Whether he deports immigrants or he who kills children, both are against life," Pope Francis said.