Iran's foreign ministry on Saturday appealed for the release of 48 of its citizens held hostage by rebels in Syria and threatened with execution one by one unless Syria's army withdraws from an area in Damascus province.
The statement, relayed by the official news agency IRNA, described the captives as "pilgrims."
The Syrian rebels, in an August 5 video, showed the Iranians and said they were members of Iran's elite Revolutionary Guards conducting a military mission in support of Syria's regime.
On Friday, a rebel commander told AFP via Internet that the regime had until late Saturday to withdraw its forces from the embattled Eastern Ghuta area of Damascus province.
"We also have other secret, military demands. If the regime does not fulfill them we will start finishing off the hostages," warned the commander, Abul Wafa, of the rebels' Revolutionary Military Council in Damascus province.
The Iranian statement, by foreign ministry spokesman Ramin Mehmanparast, said: "The hostage takers of the Iranian pilgrims in Syria as well as those supporting them are responsible for their lives."
The statement called on "international organisations to prevent such acts and to do everything to obtain the immediate liberation of all the pilgrims and Iranian nationals."
Foreign Minister Ali Akbar Salehi also called his Turkish counterpart, Ahmet Davutoglu, to ask for Turkey's help in freeing the Iranians, the Fars news agency reported. Ankara has in the past been instrumental in the liberation of other Iranians taken in Syria.
Salehi on August 8 said "retired" Revolutionary Guards members were among the hostages held by the rebels. He denied they were on active service in Syria.
The head of the Revolutionary Guards, General Mohammad Ali Jafari, told a September 16 news conference in Tehran that Iran has no "military presence" in Syria.
He said "a number of (the Guards' external special operations) Quds Force members are present in Syria and Lebanon" but purely to provide Syria's government with "counsel and advice."
Iran's foreign ministry spokesman the next day stressed that "Iran does not have any military presence in the region, especially in Syria," and said any suggestion to the contrary was "not in any way valid."
Tehran has said it is extending economic and humanitarian help to its Damascus ally.
It has repeatedly warned against foreign interference in the Syrian conflict, while accusing Turkey, Qatar and Saudi Arabia of providing military support to the rebels.
Human rights monitors have accused both the Syrian regime and rebels of committing war crimes, including extra-judicial executions.