The Britain-based Syrian Observatory for Human Rights and Syria-based activist Bebars al-Talawy said they have no information where militants took their captives after overrunning the heavily populated town of Qaryatain. Al-Talawy put the number of Christians abducted at 60.
It is believed that many of the Christians had previously fled from Aleppo province in Syria's north to seek refuge in Qaryatain.
A commentator for the Syrian Observatory for Human Rights said those abducted were wanted by Islamic State for "collaborating with the regime," and their names were on a list already prepared by the militants as they occupied the town. This has serious implications for those people and their lives could be in danger, warned the Christian rights group Middle East Concern.
Sources close to the Assyrian Orthodox metropolitan in Homs reported to Middle East Concern that while Qaryatain was under Islamic State assault, another attack centred on the town of Hawwarin, 10 km away. This is an Assyrian village and approximately 2,000 Assyrian Christians were forced to flee.
Residents of the Christian village of Sadad, 30 km west of Qaryatain, also feared for their safety.
Middle East Concern said it believes these developments suggest that Islamic State is again starting to target local Christians in Syria.
In May, masked men kidnapped Syrian Father Jacques Mourad from the Syriac Catholic Mar Elian monastery in Qaryatain.
Mourad, who helped both Christians and Muslims, was preparing aid for an influx of refugees from Palmyra when he was abducted, and his whereabouts remain unknown. Pope Francis mentioned him specifically in public prayer July 26.
While in February, Islamic State militants kidnapped more than 220 Assyrian Christians, after overrunning villages along the Khabur River in the northeastern Syrian province of Hassakeh. Only a few have been released since that time while the fate of the others remains unknown.