Saeed Jalili, a top aide to Iran's supreme leader Ayatollah Ali Khamenei, arrived in Damascus on a surprise visit on Tuesday for talks with Syrian President Bashar al-Assad, Iranian state television reported.
Jalili, the head of Iran's supreme national security council, flew in from Beirut, where he held talks on Monday with Lebanese leaders.
He was to meet Assad within the hour. After the meeting, Jalili was to give a news conference in the Syrian capital, Iranian state broadcaster IRIB said on its website.
Iran, Assad's key ally, has been increasingly critical of support by the United States, Turkey, Saudi Arabia and Qatar for the rebels fighting Syria's armed forces.
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It is also deeply concerned about the fate of 48 of its citizens abducted on the weekend by a rebel group. Tehran denies the group's claims the hostages include members of its elite Revolutionary Guards, and insists all 48 are pilgrims.
An unconfirmed report by the rebel group on its Facebook page said three of the hostages were killed on Monday in shelling by Assad's forces.
In Beirut on Monday, Jalili issued a veiled warning to countries backing the Syrian rebels.
"Those who believe that, by developing insecurity in the countries of the region by sending arms and exporting terrorism, they are buying security for themselves are wrong," he told Lebanese Foreign Minister Adnan Mansour, according to the official news agency IRNA.
"We believe that Syria's friends must help to totally stop the violence, organise national dialogue and general elections in this country, and send humanitarian aid for the Syrian population," Jalili was quoted as saying.