Pope Francis has accepted the resignation of the archbishop of Washington DC over the handling of Catholic Church sex abuse cases, praising the close ally for stepping down in the name of unity rather than fighting accusations of a cover-up.

Cardinal Donald Wuerl, 77, who was bishop of Pittsburgh between 1988 and 2006, has been under scrutiny over his handling of sexual abuse cases during that period.

He keeps the title of cardinal.

With today's resignation, Cardinal Wuerl became one of the highest ranking Catholic leaders to step aside over global accusations that the church harboured sex abusers.

And the resignation further exposes a rift within the church between Francis and a conservative wing that has opposed his efforts to be more welcoming to divorced Catholics and homosexuals.

The cardinal, one of the most prominent church figures in the United States, has been a strong defender of Pope Francis, who has been criticised by conservatives in the church, some of whom say the pope himself should resign over the sexual abuse crisis. 

In a glowing letter of support, Pope Francis made clear that he accepted Cardinal Wuerl's resignation reluctantly, at the cardinal's insistence, and believed he was not guilty of trying to conceal abuse.

"You have sufficient elements to 'justify' your actions and distinguish between what it means to cover up crimes or not to deal with problems, and to commit some mistakes. However, your nobility has led you not to choose this way of defence. Of this, I am proud and thank you," the pope wrote.

He asked Cardinal Wuerl to stay on as administrator until another archbishop could be appointed.

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The cardinal withdrew from the World Meeting of Families in Dublin in August, where he had been due to give a keynote address.

The news that he would not be coming to Dublin came three days after the publication of a Grand Jury report detailing systemic cover-ups of rapes and other sexual crimes perpetrated in six Pennsylvania dioceses by 301 clerics on at least 1,000 children over the past 70 years.

The report said Cardinal Wuerl had reassigned priests accused of abusing children.

The report found that, as a bishop in 1989, he notified the Vatican of several priests who had been accused of sexually abusing children but that over subsequent years he granted requests by some to be reassigned to other parishes or to retire early, and in one case approved a loan to assist one such priest with personal debts.

Following the publication of the report, the Diocese of Pittsburgh removed the cardinal's name from the title of a high school.

The diocese said the removal was at the cardinal's own request and that he did not want the school's old name - Cardinal Wuerl North Catholic High School - to be a distraction.

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The school was renamed North Catholic High School.

Cardinal Wuerl has defended his overall record in Pittsburgh.

He has also been accused of knowing about sexual misconduct by his predecessor in Washington, ex-cardinal Theodore McCarrick.

In July, Pope Francis accepted McCarrick's resignation - who was archbishop of Washington DC from 2000 to 2006 - over "credible" allegations he abused a teenager almost 50 years ago.

McCarrick became the first cardinal in about 100 years to be stripped of his red hat and title of "eminence".

He was ordered to retire to a life of prayer and penitence after US Church officials said as part of a separate investigation that allegations that McCarrick had sexually abused a 16-year-old boy ago were credible and substantiated.

Cardinal Wuerl has denied knowing that McCarrick had forced male adult seminarians to have sex with him years ago.