ABOARD THE PAPAL PLANE (NYTIMES) - Pope Francis said on Tuesday (Feb 5) that the Roman Catholic Church had a persistent problem of sexual abuse of nuns by priests and even bishops, the first time he had publicly acknowledged the issue.

Catholic nuns have accused clerics of sexual abuse in recent years in India, Africa and in Italy, and a Vatican magazine last week wrote about nuns having abortions or giving birth to the children of priests. But Pope Francis had never mentioned it until he was asked to comment during a news conference aboard the papal plane returning to Rome from his trip to the United Arab Emirates on Tuesday.

"It's true," Pope Francis said. "There are priests and bishops who have done that." He said that it was a continuing problem and that the Vatican was working on the issue and had suspended some priests.

"Should more be done? Yes," Pope Francis said. "Do we have the will? Yes. But it is a path that we have already begun." In answering the question, Pope Francis recalled that his predecessor, Benedict XVI, had been "a strong man" who he said had sought to remove priests who committed sexual abuse and even "sexual slavery."

Pope Francis said that he, too, wanted to move forward on the issue of the clerical abuse of nuns. "We're working on it," Pope Francis said.

He also expressed willingness to mediate in the Venezuela political crisis, if both sides wanted it, and talked about his historic trip to the United Arab Emirates.

But the pope generally avoided specifics about his conversations with officials in the United Arab Emirates about their involvement in the war in Yemen or his efforts to tackle abuse in his church.

Asked about Yemen, where his hosts are allied with Saudi Arabia in a brutal war that has brought devastation to civilians and has starved tens of thousands of children, the pope said that he had the chance in his roughly 40-hour visit to broach the issue with only a few people. But, he said, he found "goodwill" on the part of the Emirates to "start a peace process".

He also said he was not bemused by his hosts' welcoming ceremony, which included jet fighters screaming overhead. "I interpret all the gestures of welcome as gestures of goodwill," he said. "Everyone does it according to their culture."

The majority of the pope's visit was focused on interreligious dialogue with the Muslim world, and it culminated with his signing a sort of manifesto for brotherhood between the faiths between himself and Ahmed al-Tayeb, grand imam of Egypt's influential Al-Azhar mosque.

Asked about the conservative criticism that he had been Pollyannaish in his approach to the Middle East and been taken advantage of by the Muslim sheikhs, Pope Francis joked, "Not only the Muslims," and noted that his critics felt he had been manipulated by just about everyone.

But he said the document he signed was on strong theological footing.

"I want to say this clearly, from a Catholic point of view, the document has not moved a millimeter" from church teaching codified in the Second Vatican Council. He said he took the extra step of having the document vetted by a tough Dominican theologian in the papal household, who approved it. "It's not a step backward," he said. "It is a step forward."

He also made it clear that he had continued to voice his concerns about the persecution of Christians in the region - which he said his flock knew all too well - but that either "me or another Peter", meaning a successor pope, would surely visit more Muslim countries.

Earlier on Tuesday, the pope celebrated Mass at the Zayed Sports City Stadium in Abu Dhabi before roughly 135,000 Catholics, many of them migrants from India, the Philippines and South America, who had come to the Emirates to work.

The Mass, also attended by 4,000 Muslims, was the largest public celebration of a Christian rite in the history of the Muslim country, where the worship of other faiths is tolerated but is not typically done so publicly.

The next major event on the pope's schedule is a meeting with presidents of the world's bishops' conferences at the end of February in Rome to focus on a response to the global sex abuse crisis that is threatening the pope's legacy and the moral capital that is the currency of his pontificate.

The pope is less than eager to receive questions about the issue. But he mentioned, in his positive appraisal of the United Arab Emirates, that it had held a conference last year on the protection of children from online predators.

"Pedo-pornography today is a big-money industry," the pope said.