The first Catholic priest in the United States to be found guilty of covering up claims of child sex abuse has been sentenced to up to six years in prison.

Judge M Teresa Sarmina told Monsignor William Lynn, 61, that he protected "monsters in clerical garb who molested children".

The former secretary of the clergy for the Philadelphia Archdiocese was found not guilty of another charge of child endangerment and conspiracy.

Lynn, who oversaw the work of 800 priests, was convicted of covering up sex abuse allegations, often by transferring abusive priests to unsuspecting parishes.

His case, which was closely watched by the Vatican, followed a series of child abuse scandals that hit the Catholic Church in the US, as well as in Ireland and Europe.

Lynn worked for the late Cardinal Anthony Bevilacqua, the longtime archbishop of Philadelphia who died in January.

The Archdiocese of Philadelphia is the US's sixth largest with 1.5 million members.

Problems with abusive priests in the Philadelphia diocese had been flagged in a 2003 grand jury report that found church leaders failed to report abuse to the authorities.

Among Lynn's job responsibilities was investigating sex abuse claims from 1992 to 2004.

"I tried to serve God. I tried to help people," Lynn told the judge before the sentence was handed down.

Turning to the family of a former altar boy who testified that he had been assaulted by a priest assigned to a church by Lynn, the monsignor said: "I hope someday they will accept my apology."

Lynn, wearing his clerical collar, showed little emotion as he was sentenced but his defence attorney, Thomas Bergstrom, later called the sentence "grossly imbalanced".

"All of a sudden (Lynn) is being held responsible for all of the abuse that occurred over 30 to 40 years, none of which he participated in," Mr Bergstrom said.

The defence argued that Lynn tried to handle cases of paedophile priests, making a list in 1994 of 35 accused predators and writing memos to suggest treatment and suspensions.

"He is being punished for things he did and did properly," Mr Bergstrom said. He said Lynn would appeal and he was seeking bail.

Lynn had faced the possibility of a slightly longer maximum sentence of up to seven years behind bars for his conviction on a single count of child endangerment.

The Archdiocese of Philadelphia issued a statement saying "fair-minded people will question the severity" of what it called a "heavy" sentence.

"We hope that when this punishment is objectively reviewed, it will be adjusted," it said.

Noting the "public humiliation" the church has undergone, it said the church has changed since the events at the centre of the trial. "We have taken dramatic steps to ensure that all young people in our care are safe," it said.

Ahead of the sentencing, eight witnesses testified in defence of Lynn's character. One of them, Rev Joe Watson, said: "He taught me a lot about how to be a good man. He was always a role model."

District Attorney Seth Williams praised the sentence.

"I think you may have heard that he did a lot of good work as a parish priest, and I don't refute that," he said.

"The issue for us is what he did as secretary of the clergy, and the scores, the hundreds of thousands of children who were placed in harm's way as a result of a culture, a bureaucracy, that protected paedophile priests instead of protecting children."

Key to Lynn's conviction on 22 June, according to the jury foreman, was the monsignor's own testimony that he had followed the cardinal's orders to attribute priests' moves to health reasons but never to sex abuse accusations.

At his trial, Lynn testified that after a priest named Edward Avery was accused of sexually abusing a boy, Lynn reassigned him to St Jerome's Church, which also housed an elementary school.

There, according to a grand jury report describing an epidemic of abuse in Philadelphia, Avery molested the altar boy, then ten, who testified at the trial.

Avery, who is defrocked, was scheduled to go on trial with Lynn but pleaded guilty and is now in jail.

Lynn's defence attorney said his client could not have known what would happen to the altar boy at St Jerome's. "Every single priest that he investigated never ever abused again, except Avery, and we've dealt with that," Mr Bergstrom said.

Lynn's sentence was for a minimum of three years in prison, after which a state parole board will decide whether he must serve more time, depending on his behaviour and other factors.