Caroline Kennedy was reportedly among the list of vetoed candidates
The Vatican has rejected at least three possible candidates proposed by Barack Obama to serve as US ambassador to the Holy See, say reliable sources in Rome.
None of the three candidates informally proposed by the Obama administration so far is acceptable to the Pope because of their support for abortion rights.
One of the potential nominees vetoed by the Vatican is Caroline Kennedy, daughter of the former US president.
Conservative Catholics in the US had already criticised her candidacy.
They say her outspoken pro-choice views on abortion made her an unsuitable choice.
The Vatican is unhappy about President Obama's support of abortion rights and his lifting of a previous ban on embryonic stem cell research in the US.
The Vatican spokesman, Father Federico Lombardi, has described media speculation about the selection of the next US ambassador to the Holy See as "unreliable". He said that no names have yet been officially submitted to Rome by the Obama administration.
And a US official repeated the standard administration position that the White House has not announced any names for ambassadorial candidates to the Vatican.
However, the BBC's David Willey in Rome says that even the informal dismissal by the Vatican of the first names whispered in confidence within the Obama inner circle is a sign that choppy waters may lie ahead in official US-Vatican relations.
The White House may be running out of time to find a suitable future American envoy to the Pope before President Obama travels to Italy in July, when he is expected to meet Pope Benedict XVI for the first time, before or after attending a G8 summit to be held in Sardinia.
Since the US established formal diplomatic relations with the Vatican in 1984, the ambassadorial post has always been held by pro-life Catholics under both Republican and Democrat administrations.
The ambassador will replace Mary Ann Glendon, a Harvard University professor who held the post during George W Bush's presidency.