Iran’s Foreign Ministry called Saturday for the release of 48 Iranian hostages held in wartorn Syria by rebel forces.

Their captors have threatened to execute the hostages unless the Syrian government releases rebels it is holding and withdraws from the embattled Eastern Ghuta area of Damascus province by late Saturday.

The Free Syrian Army maintains that the hostages are Revolutionary Guardsmen who are in Syria to aid the Bashar Assad regime in its bloody 18-month-long crackdown on opponents. Iran maintains that the men are religious pilgrims.

According to Iranian Foreign Ministry spokesman Ramin Mehmanparast, the “defenseless pilgrims” are being used by the rebels as human shields. He added that the hostage-takers, as well as those who support them, are responsible for the lives of the Iranians.

In a statement posted by the Islamic Republic News Agency (IRNA), Iran called on international organizations “to prevent the repetition of such anti-human measures and to do everything to obtain the immediate liberation of all the pilgrims and Iranian nationals.”

The 48 Iranians were abducted by the Free Syrian Army in the vicinity of Damascus on August 4.