Islamic State fighters have abducted 220 Assyrians in northeastern Syria in recent days, activists say, as international concern grows for the minority group.
The kidnappings - more than twice as many as previously reported - have prompted thousands more Christians to flee their homes to avoid being captured by the Sunni Muslim extremists, activists said.
"No fewer than 220 Assyrian citizens were abducted by IS over the past three days from 11 villages" in Hasakeh province, the opposition British-based Syrian Observatory for Human Rights said.
"Negotiations are under way through mediators from Arab tribes and a member of the Assyrian community to secure the release of the hostages," it added.
The United Nations and the United States denounced the mass abduction of Christians - the first of its kind in the war-torn country - and demanded their release.
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"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.
The UN Security Council also condemned the abductions, demanding the hostages be immediately and unconditionally freed.
Osama Edward, director of the Assyrian Human Rights Network, said on Wednesday he believed the abductions were linked to the jihadists' recent loss of ground after US-led air raids.
"They took the hostages to use them as human shields," he said.
The jihadists, who are battling Kurdish fighters on the ground, may try to exchange the Assyrians for IS prisoners, he said.
Before the Syrian conflict erupted in 2011, there were 30,000 Assyrians in the country, among an estimated Christian population of about 1.2 million.
The Assyrians, from one of the world's oldest Christian communities, have faced an increasing threat since IS captured large parts of Syria.
