Islamic State militants appear to have killed three Christian captives in Syria and are threatening to kill 200 more held hostage if their ransoms are not paid, monitoring groups have said.

The men were captured in February when Isis overran a series of Assyrian settlements on the Khabur river in north-east Syria – villages that were populated in the early 20th century by Assyrian Christians fleeing Turkish genocide.

A video released by the group shows the men dressed in orange jumpsuits before they are shot dead. One of the men says Isis will begin executing other prisoners if the militants’ demands are not met, according to the UK-based Syrian Observatory for Human Rights.

Isis captured 253 Assyrian Christians from the area in the initial offensive, periodically releasing elderly prisoners. The militants now hold 202. Assyrian activists said Isis has demanded a ransom of $100,000 (£65,000) for each prisoner.

Isis has effectively ended centuries of coexistence between Muslims and Christians in Syria and Iraq. In the summer of 2014, after sweeping through much of northern Iraq, it evicted thousands of Chaldean and Assyrian Christians from their ancestral homes in the plains of Nineveh. In other areas Isis has demanded that Christians either convert or pay the “jizya”, a tax levied on Christians and Jews for protection during the early Muslim conquests in the 7th century.

The militants have also destroyed numerous churches and Christian shrines in their campaigns.

An Assyrian activist network said in a statement: “We condemn this latest act of barbarism in the strongest possible terms. The systematic ethno-religious cleansing of Assyrian/Syriac/Chaldeans continues. They are helpless. They are children. They are women. They are somebody’s father and brother.

“We plea and beg of the international community to intervene immediately. We have been driven out of our ancestral lands. We have been killed and crucified. The international community must act now to save lives of others kidnapped.”