Saudi King Abdullah on Sunday congratulated moderate cleric Hassan Rowhani on his election as the new president of Iran, the kingdom's Shiite rival across the Gulf.
"We are glad to congratulate you in the name of the people and government of Saudi Arabia... wishing prosperity to the people of the brotherly Islamic republic of Iran," King Abdullah told Rowhani in a letter carried by the official SPA news agency.
It said the king hailed Rowhani's views on regional cooperation and improving relations.
Sunni-dominated Saudi Arabia, a key player in regional politics, was the last Arab state in the Gulf to comment on Rowhani's win.
The leaders of the United Arab Emirates, Bahrain, Kuwait, Qatar, and Oman earlier congratulated Rowhani, their respective state news agencies reported.
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The Sunni-ruled Gulf monarchies are wary of their Shiite-dominated neighbour, mainly over its nuclear ambitions.
Rowhani is a former top nuclear negotiator who has championed more constructive engagement between Iran and world powers.
Gulf states also accuse Tehran of meddling in their internal affairs, especially over its alleged support for the Shiite-led opposition in Bahrain, where authorities quelled protests in mid-march 2011.
Bahraini security forces still continue to clash with Shiite demonstrators.
Earlier this year, Saudi Arabia said its authorities had dismantled an alleged Iranian spy ring, and in May, Bahrain said that an Iranian drone was found in the sea near Saudi Arabia.
Tehran denied being involved.
The UAE, meanwhile, accuses Iran of occupying since 1971 three islands in the Gulf that the Arab nation claims to own under an agreement signed with colonial-era Britain.