Vatican City (AFP) - Pope Francis has declined to comment on a claim that he ignored sexual abuse allegations against a senior clergyman amid speculation conservative elements in the Catholic hierarchy are using the issue to mount a "putsch" to remove the liberal pontiff.

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 a 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 Argentine pontiff by conservatives in the Church.

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

In his 11-page "testimony", Vigano, 77, who was a papal nuncio in Washington between 2011 and 2016, said Benedict XVI imposed canonical sanctions against McCarrick in the late 2000s.

McCarrick was forced to leave his seminary and live a life of penance after former Vatican ambassadors in Washington, now dead, reported him for "gravely immoral" behaviour with seminarians and priests.

- 'Homosexual networks' -

Vigano claimed Francis asked him about McCarrick when he took office in June 2013, but that the pope ignored his warnings.

He said the pope "knew from at least June 23, 2013, that McCarrick was a serial predator," adding that "he knew that he was a corrupt man, he covered for him to the bitter end".

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The pope accepted the resignation of McCarrick, now 88, in July, making him just the second cardinal ever to lose his status.

In his letter, Vigano condemned the "culture of secrecy" in the Church but also railed against "the homosexual current" he claimed was prevalent in the highest echelons of the Vatican and contributing to a "conspiracy of silence" over abuse.

"The homosexual networks present in the Church must be eradicated," he said. "These homosexual networks ... are strangling the entire Church."

US cardinals Monday defended themselves against Vigano's allegations of a cover-up.

Cardinal Joseph Tobin of Newark, a progressive, expressed "shock, sadness and consternation" at the wide-ranging allegations and Cardinal Donald Wuerl of Washington denied any knowledge that his predecessor had been either sanctioned or accused of abuse.

- 'A putsch is afoot' -

"Make no mistake. This is a coordinated attack on Pope Francis," said an editorial article on the website of the progressive National Catholic Reporter weekly.

"A putsch is afoot and if the US bishops do not, as a body, stand up to defend the Holy Father in the next 24 hours, we shall be slipping towards schism," the author Michael Sean Winters wrote.

"The enemies of Francis have declared war."

Nicolas Seneze, the Rome correspondent for the French daily La Croix, echoed that there is "a clear desire to attack Francis," telling AFP that "those who regard Francis as dangerous will stop at nothing."

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

With his more conciliatory approach to the gay community Francis had raised hopes among that he might steer the Church towards greater acceptance of homosexuality but he remains in line with traditional Church teaching on sexuality and marriage.

- Abuse scandals -

During his visit to Ireland on Sunday the pope "begged for God's forgiveness" for past clerical abuse scandals, which have badly damaged the image of the Church in the Catholic stronghold.

His trip was met with enthusiastic crowds but also protests, with about 5,000 abuse victims and supporters attending a "Stand for Truth" rally in the capital Dublin.

The Catholic Church's standing has been badly dented by the abuse scandals. Stronghold Ireland has largely shed its traditional Catholic mores, voting earlier this year to legalise abortion after approving same-sex marriage in 2015.

Multiple probes in Ireland have found Church leaders protected hundreds of predatory priests and former Irish president Mary McAleese revealed this month that the Vatican had sought to keep Church documents inaccessible to government investigators.

The abuse scandals in Ireland are part of a worldwide crisis for the Vatican.

A devastating report earlier this month accused more than 300 priests in the US state of Pennsylvania of abusing more than 1,000 children since the 1950s.