Beirut (AP) — The number of Christians abducted by the Islamic State group in northeastern Syria has risen to 220 in the past three days, as militants round up more hostages from a chain of villages along a strategic river, activists said Thursday.

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 Islamic State group in Syria. 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.

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 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 extremists have declared a self-styled caliphate in the regions of both countries that are under their control, killing members of religious minorities, driving others from their homes, enslaving women and destroying houses of worship.

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.

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 Islamic State and similar groups.

The White House condemned the attacks, saying the international community is united in its resolve to "end ISIL's depravity." ISIL is one of several alternative acronyms for the IS group.

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