ISTANBUL — Dozens of Christian civilians are missing and are believed to be captives of the Islamic State extremist group after it seized their town in central Syria this week, according to conflict-monitoring groups and the Syrian Orthodox Church.

Contact with the Christians was lost after the jihadists attacked the isolated oasis town of Qariyatain in Homs Province this week and routed the Syrian Army, taking control of the town on Thursday.

The town’s isolation and the collapse of its lines of communication made it difficult to determine the precise number of the missing and their fate, but the attack raised fears that the latest jihadist advance would be a new blow to the ancient Christian communities of the Middle East.

The spread of the Islamic State’s self-declared caliphate has already uprooted large Christian communities in parts of Iraq and Syria. While the jihadists have generally not slaughtered Christians as they have Shiites and other religious groups, they have seized their property, desecrated their churches and imposed special taxes on them, forcing them to flee.