Katsav shook hands on Friday with Syrian President Bashar al-Asad, the second time initiated by al-Asad, his office said.
Katsav said al-Asad sat one row behind him during the service. When he turned to shake hands with the nearby Swiss leader, "the Syrian president also stood there. We exchanged smiles and shook hands," Katsav told the website of the Maariv daily.
"During the prayers, according to the Christian tradition we exchanged handshakes. During this, it was the Syrian president who extended his hand to me and we again shook hands," he said.
The Iranian-born Katsav says he also spoke briefly with Iranian President Muhammad Khatami in Farsi.
Katsav said as he was leaving, "the Iranian president held his hand out to me. I shook his hand and greeted him in Farsi."
But Khatami has "strongly denied" the reports, the official Iranian news agency Irna reported on Saturday.
"These allegations are false like all the other allegations (by the Israeli media) and I have not had any meeting with a personality from the Zionist regime," Khatami was quoted as saying.
"I strongly deny shaking hands, meeting and talking to the Israeli president," Khatami told Irna after his return from Italy.
Media reports said the men conversed about Yazd, the city in central Iran where both men were born. Israel accuses Syria and Iran of backing Palestinian resistance fighters in the West Bank and Gaza Strip.
Israel has also been a leading critic of Iran's nuclear programme. Katsav also reportedly embraced Algerian President Abd al-Aziz Butaflika, Israel Radio reported. Israel does not have relations with any of the above countries.