The hardline Iranian leader will meet the king, whom he previously saw at an Islamic summit in Mecca in December 2005, after flying into Riyadh, a Saudi official said.

An Iranian diplomatic source said that Ahmadinejad, who will arrive at around 1300 GMT, will stay only a few hours in the kingdom.

Relations between the two regional heavyweights have been strained over non-Arab Iran's growing influence in Iraq and its perceived backing of Shiite militias at war with the once-ruling Sunni minority there.

Lebanon has also severely tested ties between predominantly Shiite Iran and Sunni-dominated Saudi Arabia, a key US ally which provides substantial financial aid to Beirut and which has close links with the Western-backed government of Sunni Prime Minister Fuad Siniora.

The anti-Syrian Lebanese administration has been crippled by an opposition ministerial walkout and an open-ended protest spearheaded by the Iranian-backed Shiite movement Hezbollah, raising fears of Sunni-Shiite clashes similar to the sectarian bloodbath in Iraq.

In December, King Abdullah likened the situation in the Arab world to "a powder keg waiting for a spark to explode," and Riyadh and Tehran recently began working together to reduce tensions in Lebanon.

The two oil powerhouses have also sought to contain differences over Iraq, which at one point saw Saudi Arabia accusing the United States of effectively handing the country to Iran and triggered reports -- swiftly denied by Riyadh -- of possible Saudi intervention on behalf of Sunnis.

Saudi commentators see Ahmadinejad's visit as a sign that the two countries are pooling efforts to ease regional crises at a time when Tehran is under heavy Western pressure over its nuclear ambitions and Riyadh is keen to avert a US-Iran military showdown which could destabilise the entire Gulf region.

The visit "might yield a joint initiative" to break the deadlock in Lebanon and lead to "an understanding that will ease the conflict in Iraq," analyst Anwar Eshki told AFP.

Bandar al-Aiban, who heads the foreign affairs committee of the appointed Shura (consultative) Council, said Ahmadinejad's visit indicates that Iran is looking to Saudi Arabia for help in ending the standoff over Tehran's "ambiguous" nuclear programme, which Washington sees as a cover for the pursuit of nuclear weapons despite Iranian denials.

The leading daily Al-Riyadh said a "half success" of the Saudi-Iranian summit would be "better than the continuation of crises" in regional trouble spots.

"The leaderships of the two countries should tackle the issues of sectarian conflicts" between Sunnis and Shiites, the newspaper said.

"King Abdullah and the Iranian president have every chance of success if they put the will (to do so) above all else," it added.

The daily Okaz, pointing to Iranian influence in Iraq and Lebanon, urged Tehran to help stabilise the region.

"Every Arab and every Muslim aspires to see Iran play its natural role in the region... by becoming a factor of stability and peace, which would be compatible with its status in the area," the paper wrote.

"Mr President," echoed the Saudi Gazette, "your visit in this darkest hour yet for our region offers hope" of achieving Muslim unity.

"As we people of the desert know, it's always darkest before dawn."

Ahmadinejad's moderate predecessor, Mohammad Khatami, visited Saudi Arabia in 1999 and 2002, repairing relations which had nosedived after Iran's 1979 Islamic revolution.

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