Reports: 200 Assyrians thought abducted by IS in Syria
Most of the captives were women, children and the elderly, reported the BBC on Wednesday, through community sources.
Kurdish and Christian militia are battling Islamic State militants in the area.
The kidnapping of dozens of Assyrian Christians by IS in Syria has prompted an exodus of terrified families fleeing their homes, activists said on Wednesday.
The United States condemned the mass abduction of Christians - the first of its kind in the war-torn country - and demanded the release of the hostages.
Nearly 1,000 Assyrian Christian families have fled their villages in the northeastern province of Hasakeh since Monday's kidnappings, said Osama Edward, director of the Sweden-based Assyrian Human Rights Network.
About 800 families have taken refuge in the city of Hasakeh and 150 in Qamishli, a Kurdish city on the border with Turkey, Edward said.
Most of the hostages were women, children or elderly, he added.
Edward told AFP he believed the mass abduction was linked to the jihadists' recent loss of ground in the face of US-led coalition air raids against IS that began in Syria in September.
"IS has been losing territory because of the international coalition's strikes and they took the hostages to use them as human shields," the activist said.
The militants, who are battling Kurdish fighters on the ground, may try to exchange the Assyrian Christians for IS prisoners, according to Edward.
He said the aim of the militants is to take over the Assyrian Christian village of Tal Tamer, which is located near a bridge over the Khabur river that links Syria to Iraq.
IS, which also holds swathes of Iraq, last year declared an Islamic "caliphate" in areas under its control and has committed widespread atrocities.
Assyrian Christians, who are from one of the world's oldest Christian communities, have been under increasing threat since IS captured large parts of Syria.
The militants have destroyed Christian shrines and churches in territory they control.
Last week, the group's Libyan branch released a video showing the gruesome beheading of 21 Coptic Christians, mostly Egyptians.
Edward, a native of an area of Hasakeh province where 35 Assyrian villages are located, said the militants broke into houses during the night while everyone was asleep.
Since Monday, IS has captured at least a dozen villages in the area, Edward said, including his wife's hometown of Tal Shamiram.
"When she tried to reach her uncle by telephone, a man replied and said: 'This is the house of the Islamic State'," he said.
He said the hostages were taken to Shaddadi, an IS stronghold in Hasakeh province.
The militants had been intimidating the Assyrian villagers for several weeks, he said, including by threatening to remove crosses from their churches.
"People were expecting an attack, but they thought that either the Syrian army - which is just 30 kilometres (20 miles) from there - or the Kurds or the (US-led) coalition's strikes would protect them," he added.
The United States on Wednesday condemned the abductions as "brutal and inhumane".
"ISIL's latest targeting of a religious minority is only further testament to its brutal and inhumane treatment of all those who disagree with its divisive goals and toxic beliefs," US State Department spokeswoman Jen Psaki said, using another acronym for IS.
She added: "To bring an end to these daily horrors, we remain committed to leading the international coalition to degrade and defeat ISIL."
A prominent bishop meanwhile accused Turkey of allowing militants responsible for the persecution of Syrian Christians to cross its border unchecked, while preventing Christians from fleeing the war-torn country.
Jacques Behnan Hindo, the Syrian Catholic Archbishop of Hasakeh-Nisibi, made the remarks on Vatican Radio.
"In the north, Turkey allows through lorries, Daesh (IS) fighters, oil stolen from Syria, wheat and cotton: all of these can cross the border but nobody (from the Christian community) can pass over," he said.
There were 30,000 Assyrians in Syria before the country's conflict erupted in March 2011. At that point Syria had an estimated total Christian population of around 1.2 million people.