Fighters of the Islamic State of Iraq and the Levant (ISIL) group have kidnapped dozens of Assyrian Christians in northeast Syria, according to activists.

The exact number of Assyrians missing remained unclear on Tuesday, with estimates ranging from 70 to 150.

The abductions were reported after ISIL fighters seized two Assyrian villages from Kurdish forces along the Khabur River in the province of Hassakeh on Monday.

Have they been slaughtered? Are they still alive? We're searching for any news. Assyrian woman in Beirut

The Syriac National Council of Syria, a group representing several NGOs inside and outside the country, told Reuters that it had verified at least 150 people missing, including women and elderly people.

The UK-based Syrian Observatory for Human Rights said 90 people had been abducted, while Nuri Kino, the head of an group called A Demand For Action, said between 70 and 100 Assyrians were taken captive.

About 3,000 people fled and have sought refuge in the cities of Hassakeh and Qamishli, he said, adding that his activist group based its information on conversations with villagers who fled the attack and their relatives.

"Have they been slaughtered? Are they still alive? We're searching for any news," an Assyrian Christian woman originally from the affected area who now lives in Beirut told the Associated Press.

The woman said she has been trying to find out what has become of her parents, her brother and his wife and their children, but could not reach anyone in the village.

The Assyrian Network for Human Rights in Syria said on its Facebook page that the fighters had moved the captives to the village of Umm al-Masamir on Mount Abdulaziz, about 25km south of the town of Tel Tamr. That raised fears, the network said, that ISIL could use them as human shields against Kurdish militiamen.

Kurdish offensive

Al Jazeera's Nisreen El-Shamayleh, reporting from Amman, said the kidnapping appeared to be in direct response to recent gains made by Kurdish forces in Syria's northeast.

Much of Hassakeh is divided between Kurdish and ISIL control.

Fighters from the Kurdish People's Protection Units (YPG) have been on the offensive in the province in recent days.

They have taken 24 villages and hamlets as part of an operation to try to recapture the town of Tal Hamis and surrounding areas, the AFP news agency reported. Tal Hamis lies to the east of the villages taken by ISIL.

Kino told the Associated Press that about 3,000 people fled from ISIL's onslaught in the area.

YPG forces have also been on the offensive in Raqqa province, which neighbours Hassakeh, seizing 19 villages as they advance following their recapture of the strategic border town of Kobane last month.

The Kurdish forces have been backed by US-led air strikes launched by the international coalition fighting ISIL.

Northeastern Syria is strategically important in the fight against ISIL because it borders territory controlled by the group in Iraq, where last year the armed group attacked the Yazidi community.

ISIL has destroyed churches and Christian shrines in Syria, and demanded that Christians living under its rule pay a tax known as jizya.