During a 45-minute session with the media aboard the American Airlines flight that carried Francis back to Rome after his nine-day visit to Cuba and the United States, the pontiff also indirectly lent his support to US Christians who are refusing to provide services to same-sex couples.

The most celebrated case is that of a Kentucky county clerk who spent five days in jail for refusing to issue marriage licenses to gay couples.

Conscientious objection, the pope said, is a "human right," including for government officials.

It must be respected in every legal structure," he said.

Overall, Francis said, he was impressed during his visit to the United States, his first ever, by the "warmth" of the American people, whom he called "lovable," and also by the differences in the way he was received: a "little more formal" in Washington, "a bit exuberant" in New York, and "very expressive" in Philadelphia. In fact, he coined a term in Italian to describe that New York excitement: "stralimitata" -- beyond all limits.

The pontiff said he also was affected by the piety he found.

"You could see the people pray and this struck me a lot," he said.

Francis had especially warm words of praise for Catholic nuns in the United States, saying "they've done marvelous things in education and health."

"The people love the sisters," he said.

He jokingly added, "I don't know how much they love the priests, but they love the sisters."

On other fronts, the pope:
• Vowed that the Vatican will do whatever it can to support a peace deal intended to resolve Colombia's long-running civil war.
• Rejected the idea of building barriers to try to stem the tide of the present refugee crisis in Europe.
• Insisted that his recent reform of the annulment process does not amount to "Catholic divorce."
• Repeated his desire to visit China, saying "I love China."
• Once again declared that women priests are an impossibility.

On the abuse issue, Francis was responding to a question about why he'd offered a word of comfort to the US bishops for their "courage" regarding the abuse scandals in a Sept. 23 address, when several have been accused of failing to respond adequately to abuse charges against personnel under their supervision.

"These are men of the Church, of prayer, true pastors, and they've suffered very much," Francis said of the American bishops. "I wanted to express compassion."

At the same time, Francis made it clear he did not intend the comment as a free pass for those who failed to remove abusive priests and report them to authorities.

"I didn't mean to say, 'Don't worry, this is no big deal'," Francis said, insisting that when a clergy member abuses a child, it's tantamount to an act of "sacrilege."

While the Vatican has cracked down on priests who rape and molest children in recent years, it has long been accused of turning a blind eye to the bishops who moved abusers around rather than report them to police. Francis has agreed to create a tribunal in the Vatican to prosecute these bishops for abuse of office and has accepted the resignations of three US bishops accused of mismanaging abuse allegations: Bishop Robert Finn of Kansas City-St. Joseph, and Archbishop John Nienstedt and auxiliary bishop Lee A. Piche, both of St. Paul-Minneapolis.

Despite those moves, critics charge that the Church does not yet have a clear-cut system for handling bishops charged with failing to act appropriately when abuse charges surface, or imposing consequences for those failures.

Francis said he understands how a victim or a relative of a victim could refuse to forgive the priest who abused.

"I pray for them, and I don't judge them," Francis said.

He recalled that in his previous meeting with survivors of sex abuse, in July 2014, one told him that her mother had lost her faith and died an atheist after learning that a priest had violated her child.

"I understand this woman. I understand, and God who is better than me understands," Francis said. "And I am sure that God received this woman. Because what was groped and destroyed was her flesh, the flesh of her daughter. I understand. I cannot judge someone who cannot forgive."

In response to a different question, Francis said that a priest who commits abuse is entitled to ask God's forgiveness, but will receive it only if he genuinely repents and changes his behavior.

The pontiff was also asked if he supported the right of government officials to refuse to cooperate in laws they consider unjust.

"It's a human right and a government official is a human being," he said. "One must be permitted to make an objection of conscience."

On Colombia, Francis expressed the belief that "there is good will on both sides," referring to the country's government and its leftist rebel movements, to strike a deal, but said "we still have to arrive at a definitive agreement under international law."

Francis also revealed that he has spoken personally twice to Colombian President Juan Manuel Santos to encourage reaching a resolution of the conflict.

In response to Hungary's recent decision to build security barriers to try to deter refugees from entering the country, Francis predicted they would not endure.

"All walls fall down," he said, adding that it may happen "today, tomorrow, or after 100 years." The pontiff called for building "bridges, not walls."

Francis also vigorously defended his recent decision to streamline the process for granting annulments, meaning a finding by a Church court that a union between a man and a woman, even if it included a Church wedding, was not really a marriage because it failed one or more of the tests for validity under Church law.

Francis dismissed suggestions it amounts to "Catholic divorce," saying it still involves a judicial process, and insisted that "matrimony is indissoluble ... the Church will not change this doctrine."

The pontiff also said it's "simplistic" to believe that the second round of the Synod of Bishops for the family, set to open Oct. 4, will solve the question of whether divorced and civilly remarried Catholics should be able to receive Communion. He noted that it must also face other issues, such as the inadequacy of marriage preparation efforts.

"I often think that becoming a priest takes eight years," he said, adding that priests can be returned to the lay state under certain circumstances.

"Marriage is forever, yet it requires much less preparation," he said. "Something's not right there."

Although the Vatican does not presently have diplomatic relations with China, Francis expressed a keen interest in improving ties.

"China is a great nation that brings a great culture to the world," he said. "I would like very much to go to China, I love China ... To have a country like China as a friend, with so much culture and the chance to do so much good, would be a joy."

Francis also said that the Vatican and China "have contacts and we talk."

As he has on other occasions, Francis went out of his way to praise nuns in the United States, who in recent years have been the subject of two separate Vatican investigations.

"I felt the obligation to thank them for what they've done," he said. "An important person in the American government told me in these days [during the trip] that whatever culture he has, he has [because of] the sisters."

Finally, Francis once again closed the door to women priests.

"That can't be done," he said. "After a long, long reflection, Pope John Paul II said it clearly," referring to a 1994 document of the late pope holding that the Church cannot ordain women because Christ did not include them among his apostles.

Francis insisted the ban on women priests is not tantamount to gender bias.

"Look, in the Church women are more important than men," he said. "The Church is female ... it's the spouse of Jesus Christ."

Finally, Francis was asked if the rapturous reception he received over six days in the United States cemented his status as a star.

"A pope must be the servant of the servants of God," he said, "That's a little different than a star."

Material from the Associated Press was used in this report.

John L. Allen Jr., associate editor, specializes in coverage of the Vatican