Iran says the hostages are religious pilgrims who were snatched on Saturday as they were en route to their hotels near Damascus, the Syrian capital.
But a Syrian rebel brigade publicly labeled the captives Iranian militiamen visiting Damascus on a "reconnaissance mission." The rebel brigade says it abducted the Iranians in an operation planned for several months. The rebel faction has threatened to execute the captives.
It is unclear if the rebel group has made any specific demands in exchange for the release of the Iranians.
According to unconfirmed accounts, the rebels say three of the hostages were killed this week in government shelling.
Saeed Jalili, Iran's security chief, arrived Tuesday in Damascus, where he was to meet with President Bashar Assad, Iranian media said. Iran is a close ally of Assad, whose forces are battling a rebellion now in its 17th month.
"Kidnapping innocent people is not acceptable anywhere in the world," Jalili told journalists on arriving in Damascus, reported the official Islamic Republic News Agency.
Tehran's top security man is said to be close to Iran's supreme leader, Ayatollah Ali Khamenei, adding to the importance of his trip. Jalili arrived in Syria after an official visit to Lebanon. He was expected to travel to Iraq after his stay in Syria, Iranian media reported.
Meantime, Iranian Foreign Minister Ali Akbar Salehi is scheduled to arrive in Ankara on Tuesday for talks with his Turkish counterpart, Ahmet Davutoglu.
Iran has publicly asked for help from Turkey and Qatar in securing the release of the hostages. The two nations have supported the Syrian opposition.
Meanwhile, Iran says it plans to hold a ministerial meeting in Tehran on Thursday with nations having a "principled and realistic position on Syria," Iranian media reported.
The Iranian government has not identified the 10 nations expected to participate.
Iran denies allegations that it is providing military assistance to Assad's government. Tehran has been harshly critical of aid flowing to the rebels from the United States and its allies, especially Turkey, Saudi Arabia and Qatar.
-- Ramin Mostaghim
Patrick J. McDonnell in Beirut contributed to this report.
Photo: Saeed Jalili, Iran's security chief. Credit: Nabil Mounzer / EPA