The number of Assyrian Christians abducted by the Islamic State in Syria has risen to 220 - far more than was previously feared, according to activists.

Militants are said to have rounded up more hostages from a chain of villages along a strategic river in the past three days.

After the first abductions on Monday, it was reported that 150 Assyrian Christians were seized after ISIS took control of two communities from Kurdish forces in the northeast province of Hassakeh.

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A Kurdish fighter fires an anti-aircraft weapon from the village of Tel Tawil in the direction of ISIS fighters who have seized more Assyrian Christians in the area

Christian fighters carry their weapons towards a checkpoint after launching an offensive against ISIS since 220 Assyrian hostages were seized

But now the Britain-based Syrian Observatory for Human Rights said that ISIS had picked up dozens more hostages from 11 communities near the town of Tal Tamr.

The province, which borders Turkey and Iraq, has become the latest battleground in the fight against ISIS 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 Organisation, said ISIS had raided as many as 33 villages.

Assyrian Christians board a bus and flee the town of Tel Tamr in fear after ISIS began the abductions on Monday

Assyrians load their belongings on to a pick-up truck and flee to safety. The province, which borders Turkey and Iraq, has become the latest battleground in the fight against ISIS in Syria

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.

The U.N. Security Council has 'strongly condemned' the abductions and demanded the immediate release of others abducted by ISIS and similar groups.

The White House also condemned the attacks, saying the international community is united in its resolve to 'end ISIS depravity.'

After the attacks, the jihadists set fire to a church and then installed fighters in the remains of the building, an activist network reported.

The US-led coalition fighting ISIS, which has backed Kurdish forces battling the group, then bombed the building on Monday, destroying it and killing ISIS militants inside, said the Syrian Revolution General Commission.

The Assyrian village of Abu Tina, pictured, was recently recaptured by ISIS fighters who have declared a self-styled caliphate

It comes after reports yesterday that ISIS were planning to release a video showing the Christian hostages, and threatening to kill them if US-led airstrikes against militants in Syria continued.

The kidnappings come as ISIS released a video showing militants beheading 21 Egyptian Coptic Christians in Libya earlier this month.

The mass abductions are adding to fears among religious minorities in both Syria and Iraq, who have been repeatedly targeted by the ISIS.

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.