On his first papal trip to the U.S., Benedict gave hundreds of spectators a two-handed wave as he stepped off a special Alitalia airliner that brought him from Rome. Students from a local Catholic school screamed ecstatically when the saw the pontiff, who shook hands with President Bush, first lady Laura Bush and their daughter, Jenna on the tarmac.

The pope and the president left in a motorcade a few minutes later.

On the flight to the United States from Rome, Benedict addressed the most painful issue for the Roman Catholic Church in America - clergy sex abuse. The U.S. church has paid out $2 billion in abuse costs since 1950, most of that in just the last six years.

"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 on the flight from Rome to Washington, speaking in English as he responded to questions submitted by reporters ahead of time.

Benedict pledged that pedophiles would not be priests in the Catholic Church.

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

The pope's promise failed to mollify advocates for abuse victims, however. They said the problem is not just molester priests, but bishops and other church authorities who have let errant clergymen continue to serve even after repeated allegations.

"It's easy and tempting to continually focus on the pedophile priests themselves," said Peter Isely, a board member of the Survivors Network of those Abused by Priests. "It's harder but crucial to focus on the broader problem - complicity in the rest of the church hierarchy."

Benedict's pilgrimage is the first trip by a pontiff to the United States since the case of a serial molester in Boston triggered a crisis that spread throughout the United States and beyond in 2002. Hundreds of new accusations - many dating back decades - have surfaced each year since. There were 691 new accusations in 2007 alone, according to an annual report from the U.S. Conference of Catholic Bishops.

As head of the Vatican agency that enforces adherence to Catholic doctrine, then-Cardinal Joseph Ratzinger was heavily involved in gaining Vatican approval for the reforms U.S. bishops proposed for the American church. The bishops have since released several reports analyzing the scandal and have pledged that all credibly accused priests will be pulled from public ministry.

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

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.

President Bush made the unusual gesture of greeting Benedict at Andrews Air Force Base - the first time he has welcomed 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 embryonic stem cell research.

White House press secretary Dana Perino, asked about the pope's comments regarding the clergy sex abuse scandal, said she wouldn't rule out that the topic would come up in conversation between the pope and the president.

But she added that "I don't think it's necessarily on the president's top priorities" for his agenda in talking with the pope.

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

As for the war in Iraq, Perino said, "Obviously, there were differences years back." She downplayed those, emphasizing instead a strong bond between Bush and the pope.

Benedict "will hear from the president that America and the world need to hear his message, that God is love, that human life is sacred, that we all must be guided by common moral law, and that we have responsibilities to care for our brothers and sisters in need, at home and across the world," Perino said.

A crowd of 9,000 or more is expected at the White House Wednesday to greet Benedict on his 81st birthday. Aides say he is in good health.

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.

On Friday, Benedict will address the United Nations. The day before, he will celebrate Mass at Nationals Park in Washington. His final event of the week will be another big Mass, this time in New York's Yankee Stadium on Sunday.