Pope Francis has criticised US bishops for failing to show unity in the face of a sexual abuse crisis, saying internal bickering had to end over the scandal which has decimated the credibility of the American Church.

In a long and highly unusual letter sent as US bishops started a week-long retreat to reflect on the spreading crisis, Pope Francis said the handling of the scandal showed the urgent need for a new approach to management and mindset within the Church.

"God's faithful people and the Church's mission continue to suffer greatly as a result of abuses of power and conscience and sexual abuse, and the poor way that they were handled," the pope wrote.

He added that bishops had "concentrated more on pointing fingers than on seeking paths of reconciliation".

Pope Francis has summoned senior Catholic bishops from around the world to the Vatican next month to discuss the protection of minors, in his latest attempt to come to grips with the abuse crisis which first erupted in the United States.

Ahead of that meeting, US bishops gathered on Wednesday near Chicago for seven days of prayer and spiritual reflection.

"The Church's credibility has been seriously undercut and diminished by these sins and crimes, but even more by the efforts made to deny or conceal them," the pontiff said.

He said he was so concerned by the situation that he had hoped to attend the US retreat in person, but added that he had been unable to do so "for logistical reasons".

The pope said he was so concerned by the situation that he had hoped to attend the retreat in person, but added that he had been unable to do so "for logistical reasons".

However, he sent his own preacher, Raniero Cantalamessa, to lead proceedings.

The US Church is still reeling from a grand jury reportlast year that found that 301 priests in the state of Pennsylvania had sexually abused minors over a 70-year period.

Other states have launched investigations of their own.

"The hurt caused by these sins and crimes has also deeply affected the communion of bishops, and generated not the sort of healthy and necessary disagreements and tensions found in any living body, but rather division and dispersion," the pope said.

Critics accuse Pope Francis, who became pontiff in 2013, of responding much too slowly to the sex scandals, of failing to empathise with the victims and of blindly believing the word of his fellow clergy.

But in 2018 he tried to address past wrongs, publicly admitting that he was wrong about a case in Chile and vowing that the Church would never again seek to cover up such wrongdoing.

In July, he accepted the resignation of Cardinal Theodore McCarrick, one of the US Church's most prominent figures, following allegations he had sexually abused a 16-year-old boy, while in October, Cardinal Donald Wuerl, the archbishop of Washington DC, stepped down over his handling of abuse cases.

"Combatting the culture of abuse, the loss of credibility, the resulting bewilderment and confusion, and the discrediting of our mission urgently demands of us a renewed and decisive approach to resolving conflicts," the pope wrote.

"This requires not only a new approach to management, but also a change in our mindset."