Pope Francis said on Sunday he would not respond to a former top Vatican official who accused him of having known for years of allegations of sex abuse by a prominent U.S. cardinal, calling on the pontiff to resign in an unprecedented broadside against the pope by a Church insider.

Francis, speaking to reporters on the plane returning from a trip to Dublin, said dismissively that a statement containing the accusations "speaks for itself".

In a detailed 11-page bombshell statement given to conservative Roman Catholic media outlets during the pope's visit to Ireland, Archbishop Carlo Maria Vigano accused a long list of current and past Vatican and U.S. Church officials of covering up the case of Cardinal Theodore McCarrick, who resigned last month in disgrace.

In remarkably blunt language, Vigano said alleged cover-ups in the Church were making it look like "a conspiracy of silence not so dissimilar from the one that prevails in the mafia".

"Pope Francis has repeatedly asked for total transparency in the Church," wrote Vigano, who has criticized the pope before.

"In this extremely dramatic moment for the universal Church, his extremely dramatic moment for the universal Church, he must acknowledge his mistakes and, in keeping with the proclaimed principle of zero tolerance, Pope Francis must be the first to set a good example for cardinals and bishops who covered up McCarrick’s abuses and resign along with all of them," Vigano said.

The statement, which contained no supporting documents, was the latest blow to the credibility of the U.S. Church. Nearly two weeks ago, a grand jury in Pennsylvania released the findings of the largest-ever investigation of sex abuse in the U.S. Catholic Church, finding that 301 priests in the state had sexually abused minors over the past 70 years.

On the plane returning from a trip to Dublin, reporters asked the pope about the statement, which was published by the National Catholic Register and several other conservative media outlets in the United States and Italy.

"I will say sincerely that I must say this, to you (the reporter) and all of you who are interested: Read the document carefully and judge it for yourselves," he said.

"I will not say one word on this. I think the statement speaks for itself and you have sufficient journalistic ability to reach your own conclusions," he said.