On the papal flight back to Rome, Pope Francis said all workers have a "human right" to refuse to discharge their duties, such as issuing marriage licenses to homosexuals, if they feel it violates their conscience. (Reuters)

Pope Francis broadened his warnings over the church’s sex abuse scandals Monday, saying that clergy and others who helped cover up wrongdoing are also guilty in the eyes of the Vatican.

The pope also indirectly added his voice to the charged U.S. debate over religious liberty — only hours after wrapping up his three-city visit to the United States — by expressing strong backing for those who take a stand for their spiritual views.

Francis made no specific references, but his endorsement is certain to be perceived as a response to ongoing battles over public officials’ refusals to obey laws permitting same-sex marriage.

In wide-ranging comments made during his flight back to Rome, Francis shrugged off his celebrity, expressed interest in visiting China and reflected on the huge outpouring of support during his U.S. visit — his first.

“I was surprised by the people’s warmth,” he told journalists aboard the papal plane. “They were so friendly; it was beautiful.”

Pope Francis returned to the Vatican Monday after a 10-day trip to Cuba and the United States. (AP)

During the visit, Francis met with clergy abuse victims and issued an apparent message of consolation to American bishops, noting their “courage” in confronting the scandals.

But his latest statement suggests that the Vatican could widen the scope of its investigations and punishments as the Catholic Church faces pressure for greater accountability at all levels. In June, Francis approved a special Vatican tribunal to examine bishops accused of covering up sex abuse allegations or failing to act.

“Those who covered this up are guilty,” he said Monday, in his first such statement. “There are even some bishops who covered this up. It’s something horrible.”

Francis also appeared to delve deeper into U.S. disputes by describing conscientious objection as a fundamental “right.”

He did not cite any case, but the comments are likely to be interpreted as touching on the case of Kentucky county clerk Kim Davis, who refused to issue marriage licenses to same-sex couples.

“If a person doesn’t allow an objection of conscience, they’re denying a right,” Francis said.

Moments after landing in Rome, the pope posted a Twitter message expressing “heartfelt thanks” to the American people.

“In Washington, it was a warm welcome but a bit more formal,” Francis told journalists. He described New York using the Italian phrase “stra limitata” — a superlative meaning “beyond limits.”

“In Philadelphia, very expressive. Different ways but the same welcome,” he said.

Asked about his “star” power, Francis acknowledged that he can command the world’s attention but noted that celebrity has its limits.

“Yes, in the media this is happening, but there’s another truth,” he said. “How many stars have we seen that go out and fall? It is a fleeting thing.”

He added, however, that being a “servant” of the faith “is something that doesn’t pass.”

The pope also evinced interest in visiting China — a trip that would touch on sensitive religious issues for the Vatican, which has opposed Beijing’s control over church affairs.

“China is a great nation that offers the world a great culture, so many good things,” he said. “I said once on the plane when were flying over China when we were coming back from Korea that I would very much, like so much, to go to China.”

Despite decades of international papal travel, China and Russia remain major countries that have not hosted a pontiff.

Turning back to Europe, Francis criticized the attempts of countries such as Hungary to impose border controls and barriers to block the wave of refugees and others flooding into the continent.

“You know how walls finish up?” he asked. “All walls collapse — today, tomorrow or after 100 years. Walls are not the solution. The problem remains, but with more hate.”

Read more:

The pope to Congress: We are all immigrants

Pope meets ‘crazy’ family at end of 13,000-mile road trip

Today's coverage from Post correspondents around the world