The Britain-based Syrian Observatory for Human Rights said the militants have 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 ISIS group in Syria. It is predominantly Kurdish but also has populations of Arabs and predominantly Christian Assyrians and Armenians.

ISIS 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.

Younan Talia, a senior official with the Assyrian Democratic Organization, said IS had raided 33 Assyrian villages, picking up as many as 300 people along the way. 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 ISIS-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 ISIS group.

The group has killed captives in the past, including foreign journalists, Syrian soldiers and Kurdish militiamen. Most recently, militants in Libya affiliated with ISIS released a video showing the beheading of 21 Egyptian Christians.

Today in Iraq ISIS fighters have abducted 100 Sunni Muslim tribesmen near the city of Tikrit, local tribal leaders said on Wednesday, apparently to neutralize suspected opponents before a widely expected army offensive.

The extremists could also use the Assyrian captives to try to arrange a prisoner swap with the Kurdish militias they are battling in northeastern Syria.

The Observatory said negotiations through mediators were taking place between Arab tribes and an Assyrian figure to secure the hostages’ release.

The U.N. Security Council on Wednesday evening ‘‘strongly condemned’’ the abduction and demanded the immediate release of others abducted by the ISIS and similar groups.

The Assyrians are indigenous Christian people who trace their roots back to the ancient Mesopotamians.