Islamic State (IS) has abducted dozens of Assyrian Christians from villages in north-eastern Syria, activists say.
The Syrian Observatory for Human Rights reported that at least 90 men, women and children were seized in a series of dawn raids near the town of Tal Tamr.
Some Assyrians managed to escape and made their way east to the largely Kurdish-controlled city of Hassakeh.
It comes as Syrian Kurdish fighters backed by US-led air strikes continue to advance into IS-held territory.
Hassakeh province is strategically important in the fight against IS because it borders both Turkey and areas controlled by the group in Iraq.
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Activists reported that IS fighters swept through a string of villages along the south bank of the Khabur river before dawn on Monday.
Residents of villages on the north bank fled, with about 3,000 believed to have headed for Hassakeh and Qamishli, another city to the north-east.
The militants have reportedly taken the male captives to nearby Abdul Aziz mountain, while the women are being held in the village of Tal Shamran, where activists say most of those captured came from.
The Syrian Observatory for Human Rights, a UK-based group that monitors the four-year conflict in Syria, said at least 90 Assyrians had been taken captive.
Meanwhile Nuri Kino, the head of the group A Demand For Action, which focuses on religious minorities in the Middle East, said between 70 and 100 had been abducted.
Islamic State's online radio station, al-Bayan, reported on Tuesday that its members had seized "tens of Crusaders".
Osama Edward of the Sweden-based Assyrian Human Rights Network, who has relatives in the area, told the BBC that his wife's elderly aunt and her cousin were among the hostages.
"My wife tried to call her cousin's house and there was somebody who picked up the phone and said: 'This is not Akram's house. This is the Islamic State's house'."
The Syrian Observatory also said Tal Tamr had seen heavy clashes between IS and the Kurdish Popular Protection Units (YPG) militia.
On Sunday, YPG fighters launched an offensive about 90km (55 miles) to the east, near the Iraqi border.
Backed by US-led coalition aircraft and Iraqi Kurdish Peshmerga, they quickly seized about 20 IS-held villages and forced the militants back to within 5km (3 miles) of the town of Tal Hamis, according to the Syrian Observatory. Another 10 villages reportedly fell to the YPG on Tuesday.
Christians are believed to have constituted about 10% of Syria's 22 million people before the uprising against President Bashar al-Assad began almost four years ago.
Assyrians, of whom there were about 40,000 in Syria, are Nestorian Christians and speak Syriac, a form of Aramaic, the language of Christ.
The largest concentration of Assyrians in Syria is in Hassakeh province, but there are also smaller communities in Aleppo, Homs and Damascus.
Many Assyrians are believed to have fled Syria not only to escape the conflict but also violent attacks by extremist groups like IS, which has forced Christians living in the territory it controls to either convert to Islam, pay a religious levy (jizya) or face death.
The BBC's Jonny Dymond in Beirut says the motive for the seizure of so many Assyrians not yet clear.
A few weeks ago, Assyrians in the area were instructed by IS to remove crosses from churches and pay jizya; and Assyrian militia have taken part in the Kurdish offensive against IS in the area.
Our correspondent says it may be that the Assyrian prisoners are to be used as part of a swap with the Kurdish forces.
Mr Edward said the militants had threatened to kill the captives unless the Kurdish fighters halted their offensive, although there was no confirmation from al-Bayan.