Pope Francis received the Iranian vice-president for family affairs Sahindokht Molaverdi in February and has been a prominent backer of the nuclear accord as a significant step towards stability in the region.
The last Iranian president to visit the Vatican was Mohamed Khatami, who held talks with Jean-Paul II in 1999 and subsequently attended the Polish pope`s funeral in 2005.
Paul VI (1963-78) is the only pope to have visited Iran and that was only a brief stopover on his way to Asia in 1970.
But the 1979 Islamic revolution did not lead to a break in diplomatic relations.
Iran outlaws conversions to Christianity but the country`s eastern Catholics remain free to exercise their religion.
Vatican relations with Tehran are smoother than with several Arab Gulf states seen as placing greater restrictions on Christians` religious freedom.
French cardinal Jean-Louis Tauran, president of the pontifical council for inter-religious dialogue, has made several visits to Tehran.
There is even collaboration in the unlikely area of astronomy: an Islamic-Christian seminar on the subject was organised in January at the Vatican`s observatory outside Rome.
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