Two cardinals have criticised Pope Francis's approach to the clerical sexual abuse crisis, saying "abuse of power" was not to blame but homosexuality.

"We turn to you with deep distress. The Catholic world is adrift," US Cardinal Raymond Leo Burke and German Cardinal Walter Brandmueller said in an open letter to the heads of the bishops' conferences.

"Sexual abuse is blamed on clericalism. But the first and primary fault of the clergy does not rest in the abuse of power but in having gone away from the truth of the Gospel," they said.

They blamed a corruption of the "absolute moral law", referring to homosexuality, which they described as the "denial, by words and by acts, of the divine and natural law".

"The plague of the homosexual agenda has been spread within the church, promoted by organised networks and protected by a climate of complicity and a conspiracy of silence," claimed the cardinals, who belong to the church's conservative wing.

The Catholic Church considers the practice of homosexuality to be a sin.

Cardinal Leo Burke

The letter comes a day before Pope Francis opens a historical summit at the Vatican on the protection of minors in a bid to end a scandal that has dogged the Catholic Church for decades.

Cardinals Burke and Brandmueller criticised Pope Francis, saying they were "among those who in 2016 presented to the Holy Father certain questions ... [that] have not only not had any response, but are part of a more general crisis of the faith".

They called on the head bishops "to raise your voice to safeguard and proclaim the integrity of the doctrine of the church".