Bernie Sanders announced Friday he will attend a Vatican City conference next week, touching off a long-distance dispute over whether the Democratic presidential candidate had elbowed his way into the summit.

Bloomberg News quoted a senior Vatican official alleging the Vermont senator sought the invitation, accusing him of “monumental discourtesy” in doing so.

But another senior Vatican source told Fox News that is not quite true.

Sanders did receive a written invitation dated March 30 from Monsignor Marcelo Sanchez Sorondo, chancellor of the Pontifical Academy of Social Sciences where Sanders will speak.

There are varying reports, though, over who approached whom. 

Margaret Archer, president of PASS, told Bloomberg that “Sanders made the first move, for the obvious reasons,” while accusing him of going after the Catholic vote.

But the Sanders campaign called the claim “categorically untrue.”

Sorondo, speaking to Bloomberg, apparently did not say who initiated the contact – the Vatican or Sanders. Yet later speaking with Reuters, Sorondo denied that Sanders had invited himself.

Further, he told the Associated Press he extended the invitation to Sanders because he seems to have a "real interest" in studying the papal documents issued by the pope. He did not issue invitations to the other presidential candidates.

"I don't see the other candidates quoting the pope in their campaign. I don't know if the other candidates are interested in the documents of the pope," he said.

Regardless of who made the first move, the senior source who spoke with Fox News stressed that the invite did not come from Pope Francis himself.  

The source told Fox News the Vatican is eager to stay out of the presidential campaign -- especially with pro-life Catholics expressing unease on social media that the pro-choice Sanders is visiting.

"Is that going to thrill us? No," the Vatican source told Fox News of Sanders' stance on abortion

It was not clear yet whether Sanders, the first Jewish candidate to win a presidential primary, would meet with the pope during his trip. Sanders spokesman Michael Briggs said "if the opportunity arises he would be delighted to meet with the pope" but Sanders has not received an official invitation from the Catholic leader.

"The moral imperative that (the pope) is bringing to this discussion is absolutely extraordinary and absolutely what the world needs. These are issues that I have been dealing with for years," Sanders said in a phone interview with The Associated Press.

Attendees of the Vatican conference will include Presidents Evo Morales of Bolivia and Rafael Correa of Ecuador, along with Cardinal Oscar Rodriguez Maradiaga of Honduras, a member of the academy, and Columbia University professor Jeffrey Sachs, an adviser to the United Nations on environmental and sustainability issues.

The meeting will mark the 25th anniversary of Pope John Paul II's encyclical Centesimus Annus, a high-level teaching document which advocated for economic and social justice and environmental sustainability.

Sanders will be speaking at the conference of the Pontifical Academy of Social Sciences, an advisory group comparable to a think tank that the pope has appointed to guide him on a wide range of public policy issues.

The Rev. Thomas Reese, an analyst with the National Catholic Reporter and author of "Inside the Vatican: The Politics and Organization of the Catholic Church," said it was unusual for a U.S. presidential candidate to be invited to participate in such an event in the middle of a campaign. European politicians and experts attend frequently, in part because they can more easily travel to Rome, he said.

But Reese cautioned that the invitation should not be interpreted in any way as an endorsement from the pope.

"Certainly the last thing Pope Francis wants to do is get involved in American presidential politics. He's made clear that he doesn't even want to interfere in Italian politics," Reese said.

Fox News’ Ed Henry and The Associated Press contributed to this report.