Pope Francis has sacked the head of the Vatican office that handles sex abuse cases, just days after he released Australian cardinal George Pell to return home to face charges of historical sexual assault.

The developments underscored how the Catholic Church's sex abuse crisis has caught up with Francis, threatening to tarnish his legacy over a series of questionable appointments, decisions and oversights in his four-year papacy.

Francis has declined to renew the mandate of German Cardinal Gerhard Mueller as prefect of the Congregation for the Doctrine of the Faith, the Vatican office that processes and evaluates all cases of priests accused of raping or molesting minors.

Francis named Mueller's deputy, Monsignor Luis Ladaria Ferrer, a Spanish Jesuit, to run the powerful office instead.