Speaking to reporters while flying to his first visit to the United States, the pope discussed the U.S. scandal that has uncovered more than 5,000 sexual abuse victims since 2002 and cost the church more than $2 billion.

"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," he said. "As I read the histories of those victims it is difficult for me to understand how it was possible that priests betrayed in this way. We are deeply ashamed and we will do what is possible that this cannot happen in the future."

Pope Benedict drew a distinction between priests with homosexual tendencies and those who molest children, saying, "I would not speak at this moment about homosexuality, but pedophilia ... is another thing. And we would absolutely exclude pedophiles from the sacred ministry."

The pope said church officials were working with seminaries to ensure candidates have no such tendencies.

"Who is guilty of pedophilia cannot be a priest," he said.