Vatican City (AFP) - Pope Francis on Monday said "silence and prayer" were the answer to those seeking "scandal and division", amid a barrage of attacks from ultra-conservative Catholics.

The pope has so far refused to respond to allegations made last month that he for years covered up sexual abuse allegations against a prominent US cardinal.

"With people who lack goodwill, with people who seek only scandal, who seek only division, who seek only destruction, even within the family: (there is nothing but) silence. And prayer," Francis said during a service at St Martha's, the boarding house where he stays.

Among some ultra-conservative Catholics, the pope is regarded as a dangerous progressive who is more interested in social issues than traditional Church matters.

His comments came after Archbishop Carlo Maria Vigano, a former Vatican envoy to Washington, claimed in August that Francis ignored sexual abuse allegations against US cardinal Theodore McCarrick for five years.

The timing of the letter's release -- right in the middle of Francis's landmark trip to Ireland -- immediately raised speculation about a campaign against the Argentine pontiff.

But so far, Francis has remained silent, refusing to address the allegations and saying only that Vigano's missive "speaks for itself".

On Saturday, Vigano launched a fresh attack, accusing the pope of knowing full well that he was meeting an arch-conservative opposed to gay marriage during a 2015 visit to the United States.

The Vatican said at the time that the pope met Kim Davis, a Kentucky clerk who has refused to sign gay marriage certificates, during a reception along with 'dozens of other guests' at the embassy in Washington.