Beirut (AFP) - The United States has condemned the abduction by the Islamic State group of dozens of Assyrian Christians in Syria and demanded their immediate release.

It was the first mass kidnapping of Christians in the war-torn country.

The abduction of at least 90 Christians appeared to be in retaliation for a major Kurdish offensive aimed at recapturing nearby villages, the Syrian Observatory for Human Rights said Tuesday.

"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 Islamic State.

"ISIL continues to exact its evil upon innocents of all faiths, and the majority of its victims have been Muslims," she added in a statement.

She added: "To bring an end to these daily horrors, we remain committed to leading the international coalition to degrade and defeat ISIL and to working towards a negotiated political solution that stops the bloodshed and secures a future of freedom, justice, and dignity for all Syrians."

The Britain-based Syrian Observatory for Human Rights said IS kidnapped the Assyrians on Monday after seizing two villages, Tal Shamiram and Tal Hermuz, in Hassakeh province from the control of the Kurdish People's Protection Units (YPG).

The group had no immediate details on those kidnapped, including whether women and children were among them, or where they were being held.

"The jihadists attacked the two villages in retaliation against the Kurds, who four days ago launched a bid backed by the US-led coalition to reclaim villages around Tal Hamis, also in Hassakeh province," said Observatory director Rami Abdel Rahman.

"The fighting around Tal Hamis has killed at least 132 jihadists in four days, and four YPG fighters on Tuesday alone," he added.

"A fifth man, a Westerner who had travelled into Syria to fight alongside the YPG, was also killed."

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There were just 30,000 Assyrians in Syria before the ountry's conflict erupted in March 2011, with most of them living throughout Hassakeh province.

They represent a tiny percentage of the country's overall pre-war Christian population, which numbered around 1.2 million people.

- Churches destroyed -

After IS attacked the two villages as well as the nearby town of Tal Tamr, which remains under Kurdish control, the jihadists set fire to a church there and then installed fighters in the remains of the building, an activist network reported.

The US-led coalition fighting IS, which has backed Kurdish forces battling the group, then bombed the building on Monday, destroying it and killing IS militants inside, said the Syrian Revolution General Commission.

Control of Hassakeh province is largely divided between Kurdish forces, who in some places patrol with regime troops, and IS fighters.

Since recapturing the strategic border town of Kobane in Aleppo from IS fighters on January 26, YPG forces have taken dozens of nearby villages.

They have also seized 19 villages from IS in Raqa, where the jihadist group has its de facto capital, and another 30 villages and hamlets in Hassakeh.

The Kurdish advances have been aided by the US-led air strikes, including a series in Hassakeh on Monday that killed at least 14 IS fighters, the Observatory said.

The mass IS abduction of Assyrians appeared to be the first of its kind in Syria, but the group has become infamous for its abuses, including the mass kidnapping of minority Kurdish Yazidis in Iraq.

It also abducted dozens of Kurdish students in Syria last year, freeing them only after months in captivity.

The group has destroyed Christian shrines and churches in the territory it controls in Syria and Iraq, and demanded a tax known as jizya from Christians who remain in its self-declared Islamic "caliphate".

It regularly refers to Christians as "crusaders," and has carried out brutal executions of foreigners held hostage in Syria.

Last week, the group's Libyan branch released a video showing the gruesome beheading of 21 mostly Egyptian Coptic Christians.

Pope Francis expressed "extreme sadness" over those killings.

The pontiff has frequently warned of the plight of the dwindling number of Christians in the Middle East.