The Vatican has clarified that Pope Francis did not personally invite U.S. Democratic presidential candidate Bernie Sanders to speak at an event that will be hosted by the Vatican's Pontifical Academy of Social Sciences.

"The invitation was made on behalf of the Pontifical Academy of Social Sciences, not by Pope Francis," Vatican spokesman Father Federico Lombardi told The Daily Beast.

"There is no expectation that the pope will meet Mr. Sanders," Lombardi added.

A senior papal official earlier clarified reports that Sanders invited himself to speak at the Vatican event set for April 15-16.

Monsignor Sanchez Sorondo, a close aide to Pope Francis and chancellor of the Pontifical Academy of Social Sciences, said it was his idea to invite Sanders.

Sorondo was responding to criticisms from pro-life Catholics who wondered why the Vatican invited Sanders to speak at a Vatican event on April 15, considering that his position on abortion is at extreme odds with Catholic belief on the sanctity of human life.

Margaret Archer, president of the academy, earlier accused Sanders of committing "monumental discourtesy'' by publicising the Vatican invite.

"Sanders made the first move, for the obvious reasons,'' Archer told Bloomberg News.

But Sorondo, a senior to Archer, contradicted her report and defended the invitation, saying "I invited him with her (Archer's) consensus.''

An invitation to Sanders dated March 30 that was emailed to Reuters was reportedly signed by Sorondo and also included Archer's name.

The conference marks the 25th anniversary of Pope John Paul II's encyclical Centesimus Annus, which sets out the Church's teaching on the social order and the place of economics within that order.

In a press release, Vatican conference organisers said the conference would hold a "serious academic discussion" on how the Church can best engage with the world on social issues in the coming years and decades, according to LifeSite News.

The internal dispute in the Vatican academy over Sanders' invitation ignores the important question regarding why it was agreed to invite Sanders to address the conference in the first place, just days before an important political contest in the U.S., LifeSite News reports.

This is also compounded by the fact that Sanders believes in abortion-on-demand at any stage, for any reason, paid for by tax dollars, the online pro-life news outlet adds.

Sanders meanwhile told reporters that he was "delighted to have been invited by the Vatican to a meeting on restoring social justice and environmental sustainability to the world economy,'' reports say.