Beirut and Amman, Reuters—Kurdish militia pressed a big offensive against Islamic State of Iraq and Syria (ISIS) in northeast Syria on Wednesday, cutting one of its supply lines from Iraq, as fears mounted for dozens of Christians abducted by the hardline group that recently beheaded 21 Egyptian Copts.

The Syriac National Council of Syria says ISIS seized 150 Assyrian Christians from villages in Al-Hasakah province in a mass abduction coinciding with the offensive in the same region by Kurdish forces backed by US-led air strikes.

Hundreds more Christians have fled to the two main cities in Al-Hasakah, according to the Syriac council and the Syrian Observatory for Human Rights, which is tracking the conflict.

ISIS has killed members of religious minorities and Sunni Muslims who do not swear allegiance to its self-declared “caliphate”. The group last week released a video showing its members beheading 21 Egyptian Coptic Christians in Libya.

The abductions in Syria follow advances by Kurdish forces against ISIS in areas of the northeast near the Iraqi border—an area of vital importance to the group as one of the bridges between land it controls in Iraq and Syria.

“They want to show themselves strong, playing on the religion string, at a time when they are being hit hard,” said Rami Abdulrahman, who runs the British-based Observatory, speaking by telephone.

The Syrian Kurdish YPG militia, backed by US-led air strikes, last month drove ISIS from the Syrian town of Kobani, since then further signs of strain have been seen in the group’s ranks.

The Assyrian Christians were taken from villages near the town of Tel Tamr, some 12 miles (20 kilometers) to the northwest of the city of Al-Hasakah. There has been no word on their fate. There have been conflicting reports on where the Christians have been taken.

“These were peaceful villages that had nothing to do with the battles,” said Nasir Haj Mahmoud, a Kurdish official in the YPG militia in northeastern Syria, speaking by telephone from the city of Qamishli.

Some Christians are fighting under the umbrella of the YPG in Al-Hasakah province, but not in that area, he added.

The new Kurdish offensive launched at the weekend was focused on dislodging ISIS from areas some 60 miles (100 kilometers) further to the east, including Tel Hamis, a town that is one of its strongholds.

The Observatory said at least 132 ISIS fighters had been killed in the fighting since February 21. Mahmoud, the Kurdish official, said seven members of the Kurdish YPG militia had been killed, including one foreigner.

In a telephone interview from the city of Qamishli, he said the YPG had cut a main road linking Tel Hamis with Al-Houl, a town just a few miles from the Iraqi border.

“This is the main artery for Daesh,” he said, using an Arabic acronym for ISIS. The Kurdish YPG militia had seized more than 100 villages from ISIS in the area, he added.

“We believe we will finish the battle of Tel Hamis in this campaign,” he added.

The offensive underlines the emergence of the well-organised Syrian Kurdish militia as the main partner for the US-led alliance against ISIS in Syria.

Mainstream rebels fighting President Bashar Al-Assad have mostly been eclipsed by jihadists, complicating a US plan to train and equip Syrian opposition forces to fight ISIS.

Washington has shunned the idea of partnering with the Damascus government, seeing Assad as part of the problem. Syrian government forces, waging a separate campaign against ISIS, have made advances against the group in Al-Hasakah in recent weeks.

The latest fighting in Al-Hasakah is one piece of the Syrian war that is about to enter its fifth year and is being fought by an array of forces on multiple frontlines.

Government forces and allied militia are also battling other insurgents including mainstream rebels and the Al-Qaeda-linked Al-Nusra Front in western areas that are mostly under state control.

A large offensive by government forces backed by the Lebanese group Hezbollah earlier this month made swift progress in the south before slowing, while an attempt to encircle rebel-held areas of the northern city of Aleppo last week was repulsed, according to the Observatory.

The US-based Human Rights Watch accused the Syrian government on Tuesday of carrying out hundreds of indiscriminate aerial attacks in the past year, most with barrel bombs, in defiance of a United Nations Security Council demand to stop.