The militants began abducting Assyrian Christians on Monday, when they attacked a cluster of villages near the town of Tal Tamr in Hassakeh province, according to the British-based Syrian Observatory for Human Rights, a pro-opposition monitoring group based in Britain.
The province, which borders Turkey and Iraq, is predominantly Kurdish but also has communities of Arabs, Assyrians and Armenians.
Islamic State now controls 10 Assyrian villages in the area and negotiations are under way through tribal mediators to try to secure the release of the captives, the Observatory said in a statement.
Also Thursday, a video surfaced on the Internet that appears to show Islamic State fighters smashing ancient statues at a museum in the northern Iraqi city of Mosul and a nearby archaeological site. A bearded man tells the camera that the artifacts are idols that must be destroyed.
The extremists have declared a self-styled caliphate, or Muslim empire, in parts of Iraq and Syria. Members of religious minorities have been attacked and killed, their homes destroyed and women enslaved. Thousands have been driven from the area.