Iran's foreign minister on Tuesday sought help from the United Nations to free dozens of Iranians captured in Syria and Libya.
Foreign Minister Ali Akbar Salehi stressed his government's "deep concern" at the fate of the 48 Iranians seized in Syria and another seven Iranian Red Crescent workers held in Libya, in a letter to UN chief Ban Ki-moon.
The letter highlighted reported comments by Syrian rebels that three of the Iranians had been killed in government shelling.
"According to the same sources, the hostage takers have threatened to kill the remaining captives in the coming hours," it said.
Salehi's letter was released by Iran's UN mission. The United Nations confirmed that the Secretary General has received the letter.
"I would like to seek the cooperation and the good offices of your excellency for securing the release of these hostages," Salehi wrote.
"The kind cooperation of the relevant United Nations offices in responding to this request of government and the families of the hostages will be highly appreciated," he added.
The 48 Iranians were taken hostage on Saturday as they traveled by bus to the airport in Damascus. Iranian authorities say they are religious pilgrims.
A Syrian rebel group, Al-Baraa Brigade of the Free Syrian Army, claimed on its Facebook page that "three of the Iranian prisoners were killed in fierce shelling" by the army on Monday.
The group said the 48 are members of Iran's Revolutionary Guard and would be executed if the government shelling continued.
The seven members of the Iranian Red Crescent were kidnapped by gunmen in the Libyan city of Benghazi on July 31.
"Iran holds the hostage takers responsible for any loss of lives or injury to the hostages," Salehi wrote. Iran's foreign ministry has also said it holds the United States responsible for the lives of the hostages in Syria.
Iran "calls for the immediate release of its abducted nationals and is of the view that using the hostages as human shields violates the international law and human rights of these innocent civilians," the letter said.