In Iraq, the IS extremists released a video purportedly showing militants using sledgehammers to smash ancient artifacts in Iraq's northern city of Mosul, describing the relics as idols that must be removed.
This week's abductions of the Christian Assyrians in northeastern Syria is one of the largest hostage-takings by the Islamic State since their blitz last year that captured large swaths of both Syria and Iraq last year.
The Britain-based Syrian Observatory for Human Rights said the militants picked up dozens more Christian Assyrians from 11 communities near the town of Tal Tamr in Hassakeh province.
The province, which borders Turkey and Iraq, has become the latest battleground in the fight against the Islamic State group in Syria.
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It is predominantly Kurdish but also has populations of Arabs and predominantly Christian Assyrians and Armenians.
IS began abducting the Assyrians on Monday, when militants attacked a cluster of villages along the Khabur River, sending thousands of people fleeing to safer areas.
It was not possible to reconcile the numbers, and the fate of the hostages remained unclear.
State-run news agency SANA and an Assyrian activist group, the Assyrian Network for Human Rights in Syria, said the group had been moved to the IS-controlled city of Shaddadeh, a predominantly Arab town south of the city of Hassakeh.
The Observatory, however, said they were still being held in nearby Mt Abdulaziz.
The mass abduction added to fears among religious minorities in both Syria and Iraq, who have been repeatedly targeted by the Islamic State group.
The group has killed captives in the past, including foreign journalists, Syrian soldiers and Kurdish militiamen. Most recently, militants in Libya affiliated with IS released a video showing the beheading of 21 Egyptian Christians.