Archbishop Carlo Maria Vigano, a former Vatican envoy to the United States, on Saturday said he had told Francis of the allegations 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 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 against the Argentinian pontiff by conservatives in the Church.

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