Some 250 Syrians were missing and feared kidnapped on Thursday after the ISIS group attacked a cement factory in an advance against government positions east of Damascus.
The jehadis launched the offensive after suffering a series of setbacks at the hands of regime troops in recent weeks including the loss of the ancient city of Palmyra, which officials said residents would start returning to on Saturday.
The fresh fighting came ahead of a new round of peace talks due next week in Geneva following a ceasefire between the regime and non-jihadist rebels that has allowed Syrian forces to focus on fighting ISIS.
After being pushed out of Palmyra on March 27, ISIS launched the fresh assault this week near the town of Dmeir, some 50 kilometres (30 miles) east of Damascus.
Residents said ISIS attacked the cement factory outside the town on Monday and that about 250 employees had gone missing.