Arriving in the US for a tour aimed at healing the wounds from a decades-long sex scandal, Pope Benedict XVI has expressed regret for cases of sexual abuse carried out by Catholic priests.

The pontiff was given an unprecedented welcome by President George W Bush, who was on hand with his wife, Laura, and daughter, Jenna, at Andrews Air Force base near Washington to greet the 80-year-old.

No visiting head of state has ever been welcomed at the airport by a US president.

Standing at the top of a red carpet rolled out for the occasion, Benedict clasped both the president's hands in his as an enthusiastic crowd of onlookers waved small, yellow and white Vatican flags and cheered loudly.

The joyous welcome contrasted sharply with the paedophile priest scandal that has rocked the US church, and which the pontiff said during the flight to Washington has made him feel "deeply ashamed".

"The Church will do everything it can to heal the wounds caused by paedophile priests" and ensure "events of this kind are no longer repeated," he told reporters on the specially chartered plane.

'Chief enforcer'

"The church must absolutely exclude paedophiles from the sacred ministry. Paedophiles cannot be priests... I insist absolutely on this incompatibility."

But one former Benedictine priest said the remarks were too little, too late.

"It's a great public relations statement, but it's 25 years too late," said Patrick Wall, who left the church 10 years ago over the child sex scandal.

"As chief enforcer for the Congregation of the Doctrine of the Faith for 24 years, he had the power and the jurisdiction to stop this, but he did nothing.

"He needs to have the power and the moral fortitude to take the bishops, priests and deacons who offend against children out of ministry."

Predator priests

Pope Benedict will address the sex scandal that has left the US church financially strapped and morally battered at a meeting in New York's St Patrick's Cathedral with Catholic clergy.

But he is not expected to grant an audience to victims of predator priests.

The largest group of victims, the Survivors Network for those Abused by Priests (SNAP), called for strong actions, not mere words, from the pope.

"We're way beyond the point at which an apology, a nice gesture, a few soothing words and promises, will be meaningful," SNAP said in a statement.

The scandal, which SNAP estimates has affected hundreds of thousands of children, was far from over, said Mr Wall.

21-gun salute

"There were 51 new credible child molestation accusations against priests and bishops in the US last year. The conflict is continuing," said the former 'cleaner' for the Benedictines -- a priest sent to restore calm in a parish after a predator priest has been in ministry.

The pope was addressing the paedophilia scandal because of the deep financial impact it has had on the church, Mr Wall said.

"The church has lost over US$3billion in settlements and in the next 10 years will lose another US$3billion if things don't change. The church is scared," he said.

The official welcome ceremony for the pope takes place on Wednesday - the pope's 81st birthday - at the White House, where he will receive a 21-gun salute in front of several thousand well-wishers.

Mr Bush and the pope will then hold "frank and open" discussions on a range of issues, including the war in Iraq, White House spokeswoman Dana Perino said.

Ground Zero visit

"I think obviously that there were differences, years back" on the March 2003 US-led invasion of Iraq, which the Vatican opposed, Ms Perino told reporters.

The two leaders also will discuss "shared values of human rights, and the importance of fighting terrorism, and also promoting religious tolerance, especially when there are religious minorities.

"The president will thank Pope Benedict for deciding to go and visit Ground Zero and pay his respects there... that's a very important gesture," she said.

Benedict will hold a mass for 48,000 in Washington Thursday before going to New York to visit the scene of the September 11, 2001 attacks on the World Trade Center and hold another huge mass at Yankee Stadium.

He will also address the UN General Assembly, where he is expected to make a plea for world peace.