Plan to evacuate militants from southern Damascus on hold
Safety issues and the death of the head of Jaysh al-Islam rebel group have been blamed for the delay.
IS militants and other rebel group fighters were expected to be transported by buses from the districts of Qadam, Hajar al-Aswad and the besieged Palestinian camp of Yarmouk, strategically located less than 10 km [6.2 miles] from the south of the capital Damascus.
"Jaish al-Islam was supposed to provide safe passage through areas east of Damascus for the buses heading to Raqa," a source told AFP on Saturday.
"About 1,200 people were supposed to leave today, but the death of Zahran Alloush means we are back to square one," the source said.
He said buses standing by to transfer the evacuees were left empty and "the plan was on hold until Jaish al-Islam reorganises itself".
Buses were set to transfer the fighters and their families to other IS group and rebel strongholds across the country, including Bir al-Qasab in Reef Dimashq, Homs and Raqqa, according to the Syrian Observatory for Human Rights [SOHR].
The planned deal highlights increasing efforts by the UN and foreign governments to bring about local ceasefires and safe-passage agreements as a step closer towards ending Syria's civil war.
The Syrian regime is to participate in the Geneva peace talks asserted Syria's foreign minister Walid al-Moualem on Thursday.
Syrian opposition groups had met in Riyadh earlier this month to lay out a collective framework for the upcoming negotiations with Assad's government.
More than 250,000 people have died since protests against Assad's regime erupted in March 2011, and millions more have fled their homes.