Rome, Washington  - En route to a six-day US visit, Pope Benedict XVI on Tuesday expressed shame over the sexual-abuse-by- priests scandal in the United States, in which child molestation by priests in US parishes went unaddressed for decades.

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 told reporters aboard his plane en route to Washington, The New York Times reported online.

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. Their mission was to give healing, to give the love of God to these children. We are deeply ashamed and we will do what is possible that this cannot happen in the future," he said.

Benedict earlier departed from Rome's Fiumicino airport to journey to the United States - his first visit there as pontiff.

We hope that we can do, and we have done and will do in the future, all that is possible to heal this wound," he was quoted as saying.

The pope made a distinction between homosexuality and pedophilia and said the church would "absolutely exclude pedophiles from the sacred ministry" and try to address the issue in its seminaries.

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

A group of survivors of sexual abuse by priests on Monday called on the United Nations to examine the issue, claiming the Vatican was in violation of the UN Charter on the Protection of the Rights of Children.

The Survivors Network of those Abused by Priests (SNAP) has called for a meeting with pontiff during his US visit, but received no reply. It expected however that Benedict would address the issue and possibly meet with a group of carefully chosen victims.

Benedict left Rome on an Alitalia Boeing 777 jet at 12:15 pm (1015 GMT) and was expected to arrive at Washington's Andrews Air Force Base at 4 pm (2000 GMT), according to a Fiumicino spokesman.

President George W Bush planned to greet Benedict on the pontiff's arrival. It would be the first time Bush has met an arriving head of state's plane at the base.

The two are scheduled to meet again on Wednesday for talks at the White House, in only the second such meeting between a president and a pontiff at the presidential residence.

Benedict is also set to address the United Nations General Assembly in New York on April 18, and visit Ground Zero, the site of the September 11, 2001, terrorist attacks.

The pontiff's US visit is scheduled to last through April 20.

Since the sex scandal first came to light in 2002, the Catholic Church has paid out hundreds of millions of dollars to the victims who were children when the violations occurred.

In the United States Roman Catholics number around 70 million, the third largest concentration in the world after Brazil and Mexico.

Meetings are also planned between the pontiff and leaders of other religious faiths - Judaism, Buddhism, Hinduism and Jainism - and with leaders of several other Christian denominations in the United States.(dpa)