Pope Francis has declined to comment on a growing scandal after a former Vatican official claimed the pontiff ignored sexual abuse allegations against a senior clergyman and called on him to resign.

Archbishop Carlo Maria Vigano, a former Vatican envoy to the United States, said he had told Francis of the claims against prominent US cardinal Theodore McCarrick in 2013.

But rather than punish McCarrick, who was forced to resign last month, Vigano said Francis had lifted sanctions imposed on him by his predecessor Pope Benedict XVI.

"Corruption has reached the very top of the Church's hierarchy," Vigano said in an eleven-page letter published on Saturday in the National Catholic Register and several conservative US Catholic publications.

But the pope refused to address the allegation on Sunday.

"I will not say a word about that. I think that the communique speaks for itself," Francis said on his plane as he flew back from Dublin to Rome.

The timing of the letter's release -- right in the middle of Francis's landmark trip to Ireland -- has raised speculation of a campaign by conservatives in the Church against the liberal pontiff.

Francis told journalists to "read the communique attentively and make your own judgement," referring to Vigano's letter.

"You have sufficient journalistic capacity to draw conclusions," he went on.