AFP/ Hasakeh

Islamic State group jihadists have abducted 220 Assyrian Christians in northeastern Syria in recent days, a monitoring group said on Thursday, 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 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," the Britain-based monitoring group said.

Many of the abductees are said to be women, children or elderly.

The United States and United Nations denounced the mass abduction of Christians -- the first of its kind in the war-torn country -- and demanded their release.

"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," State Department spokeswoman Jen Psaki said, using another acronym for IS.

Her comments were echoed by US National Security Council spokeswoman Bernadette Meehan.

"The international community stands united and undeterred in its resolve to bring an end to ISIL's depravity. The United States will continue to lead the fight to degrade and ultimately defeat ISIL."

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, told AFP on Wednesday he believed the abductions were linked to the jihadists' recent loss of ground in the face of 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 Syria's civil war 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.

In Libya, an IS branch last week released a video showing the beheading of 21 Coptic Christians, mostly Egyptians.

- 'Mutual interest' -

In Washington, Secretary of State John Kerry said the United States and Iran had a "mutual interest" in defeating IS but said the long-time foes were not cooperating to do so.

"They are totally opposed to ISIL and they are in fact taking on and fighting and eliminating ISIL members along the Iraqi border near Iran and have serious concerns about what that would do to the region," Kerry told lawmakers.

"So we have at least a mutual interest, if not a cooperative effort."

Kerry, who has been pivotal to the US drive to strike a deal to curb Iran's nuclear ambitions, said Washington had not asked Tehran to get involved in the fight against IS.

Jihadist sites meanwhile launched a social media campaign in support of IS's self-proclaimed "Islamic caliphate" with new threats against the West -- in particular Britain, France and the United States.

Syria's war began in March 2011 as peaceful pro-democracy protests against President Bashar al-Assad but escalated into a brutal civil war that brought foreign jihadists flocking to the country.

France's Prime Minister Manuel Valls on Thursday "condemned with the greatest strength" a decision by three French lawmakers to meet Assad, whom he described as "a butcher".

"For parliamentarians to go without warning to meet a butcher... I think it was a moral failing," Valls said after the cross-party group made an unofficial trip to Damascus on Wednesday.