Pope Leo on Monday defended his decision to vocally oppose assisted dying initiatives while in France, acknowledging he may have entered the political fray two months after the country approved such a measure, but wanted to share the Catholic Church's viewpoint.

"If that's politics, I did that, but it was not to get involved in French politics, it was to teach what the Church really believes," the first US pope told journalists on his flight back to Rome from his four-day visit to France.

Leo criticised assisted-dying measures several times during his three-city tour. On Sunday he told healthcare workers in Lourdes, France that their job is "to care for a suffering person, not to eliminate that person."

France's new law, which created a legal right to die for those with incurable illnesses after years of intense debate, drew heated opposition from local Catholic bishops.

Leo said on Monday he did not think he crossed any lines between Church and state.

"When I went to France, I went primarily to preach the Gospel," he said.

The Catholic Church teaches that all human life is sacred and opposes abortion, assisted dying and the death penalty.