ABOARD THE PAPAL PLANE - Pope Francis publicly acknowledged on his return to Rome from a historic trip to the United States that some U.S. bishops "covered up" child sex abuse by Catholic clergy.

"Those who have covered up these things are guilty," the pope said during the 45-minute in-flight news conference Sunday night. "And some bishops covered this up, and it is a very ugly thing."

Speaking in Italian and Spanish, Francis also said conscientious objection was "a human right" and he praised American nuns, calling them "great women."

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His comments capped off a six-day U.S. visit that saw hundreds of thousands of people wait for hours in the streets for a fleeting glimpse of the pontiff dubbed "the people's pope." During his appearances in Washington, New York City and Philadelphia, Francis stressed family values, respect for immigrants and taking care of the environment.

On Sunday, his final day in America, the pope apologized directly to abuse victims and vowed before hundreds of bishops from around the world to hold accountable both perpetrators and those within the church who shield them.

Some comforting words he gave bishops earlier in the trip during a meeting in Washington -- and which some sex abuse victims groups criticized -- were not meant to say, "Don't worry, that was nothing," Francis said.

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Instead, "I felt the need to express compassion because something really terrible happened. And many of them suffered who did not know of this," he said.

Some Vatican experts said the comments on board the flight appeared to be the first time Francis publicly and directly said some U.S. bishops covered up the sex abuse scandal.

"This is the first time the pope has said out loud and in his own name that the bishops have covered up the crime of the sex abuse, and has linked that to a promise that there will be accountability not just for the crime but for the cover-up," said longtime Vatican analyst John Allen, who was aboard the plane.

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During the flight, Francis also was asked about the controversial case of a county clerk in Kentucky who spent five days in jail for refusing to issue marriage licenses to same-sex couples.

"I can say that freedom of conscience is a right, it is a human right," Francis said. "Freedom of conscience must be respected in every judicial structure."

Francis had high praise for U.S. nuns, who had been under an investigation launched by his predecessor, Benedict XVI, but which Francis ended this year.

He said the nuns have "done marvels in the field of education, in the field of health" and that one high-level U.S. official told him, "The education I have, I owe above all to the sisters."

"The people of the United States love the sisters," he said. "I don't know how much they may love the priests," he added, jokingly, "but they love the sisters, they love them so much. They are great. They are great, great, great women."

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"I felt obligated to thank them for what they've done," he said.

The pope said that when he leaves a country after a visit, "I see the faces of so many people. I get the urge to pray for them and say to the Lord, 'I came here to do something, to do good, perhaps I have done wrong, forgive me but protect all those people who saw me, who thought of what I said, who heard me, even those who have criticized me, all of them.' That is what I feel."