Pope Benedict XVI said Tuesday he was "deeply ashamed" of the clergy sexual abuse scandal that stained the U.S. Catholic Church and pledged to work to make sure pedophiles do not become priests.

Benedict was answering questions submitted in advance by reporters aboard a special Alitalia airliner as he was flying from Rome to Washington to begin his first papal pilgrimage to the United States.

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"It is a great suffering for the church in the United States and for the church in general and for me personally that this could happen," Benedict said. "It is difficult for me to understand how it was possible that priests betray in this way their mission ... to these children."

"I am deeply ashamed and we will do what is possible so this cannot happen again in the future," the pope said.

The leader of the world's 1 billion Catholics pledged that pedophiles would not be priests in the Roman Catholic Church.

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"We will absolutely exclude pedophiles from the sacred ministry," Benedict said, speaking in English. "It is more important to have good priests than many priests. We will do everything possible to heal this wound."

Benedict's pilgrimage was the first trip by a pontiff to the United States since the scandal involving priests' sexually abusing young people rocked U.S. dioceses in the early 2000s and triggered lawsuits that have cost the church hundreds of millions of dollars in settlements.

Pedophilia is "absolutely incompatible" with the priesthood," Benedict said.

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Peter Isely, a board member of the Survivors Network of those Abused by Priests, said the pope should establish child protection policies for the worldwide church. He said church leaders who fail to discipline predatory priests should be disciplined themselves.

The Vatican had invited reporters who were to fly aboard the plane to submit written questions. Vatican officials then selected four questions to be read by the journalists to the pontiff aboard the plane.

Benedict described his pilgrimage as a journey to meet a "great people and a great church." He spoke about the American model of religious values within a system of separation of church and state.

From a presidential welcome, to two Masses at baseball stadiums, to a stop for prayer at ground zero in New York, Benedict will get a heavy dose of the American experience.

U.S. President George W. Bush planned to make the unusual gesture of greeting him at Andrews Air Force Base — the first time the president has greeted a foreign leader there.

The pope said he will discuss immigration with Bush, including the difficulties of families who are separated by immigration.

While the pope and Bush differ on such major issues on the Iraq war, capital punishment and the U.S. embargo against Cuba, they do find common ground in opposing abortion, gay marriage and stem cell research.

White House press secretary Dana Perino, asked about the pope's comments about the clergy sex abuse scandal, said she would not rule out that the topic would come up in conversation between the pope and the president, but that it wasn't necessarily a top priority on Bush's agenda for his meeting with Benedict.

Perino said the two leaders would likely discuss human rights, religious tolerance and the fight against violent extremism.

Benedict will give a speech at the United Nations during his second, New York leg of the six-day trip.

"The Catholic Church can only succeed as a peacemaker if certain universal values are shared by all," Benedict said, answering a question in Italian. He did not elaborate.

Benedict will turn 81 on Wednesday, although he seems spry and aides pronounce him in good health.

A crowd of up to 12,000 is expected Wednesday at the White House for the pope's official visit to the American president. The White House is planning a gala dinner that evening, when it says Benedict will be attending a prayer service with American bishops.

After making little headway in his efforts to rekindle the faith in his native Europe, the German-born Benedict will be visiting a country where many of the 65 million Catholics are eager to hear what he says.

A poll released Sunday by the Center for Applied Research in the Apostolate at Georgetown University found eight in 10 Catholics are somewhat or very satisfied with his leadership.

Benedict is expected to stress the importance of moral values and take on what he sees are the dangers of moral relativism — that is, that there are no absolute rights and wrongs.

Benedict's homily at New York's St. Patrick's Cathedral to a gathering of clergy will be watched for more comments about his response to the scandal.

While in New York, he will also visit the Park East synagogue, part of his efforts for close relations with Jews.

He will celebrate Mass at Nationals Park in Washington and Yankee Stadium in New York, his last major event of the trip.

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