Syrian-Kurdish forces have continued a major offensive in northern Syria against the Islamic State of Iraq and the Levant (ISIL), cutting off one of its supply lines from Iraq, as fears mounted for dozens of Christians abducted by ISIL in the area.

At least 90 Assyrian Christians were seized on Monday from villages in Hassakeh province, according to the Syrian Observatory for Human Rights, after the start of the offensive in the same region which began at the weekend.

The Syriac National Council of Syria put the figure of those abducted as high as 150. Hundreds more Christians have fled to the two main cities in Hassakeh province, according to the Council and the Observatory.

Discussing the timing of the abductions, Rami Abdulrahman, who runs the UK-based Observatory, said: "They [ISIL] want to show themselves strong, playing on the religion string, at a time when they are being hit hard."

The new Kurdish offensive launched at the weekend was focused on dislodging ISIL from areas about 100km further to the east, including Tel Hamis, a town that is one of its strongholds.

The latest fighting in Hassakeh is just 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.

Last month, the Syrian Kurds, backed by US-led air strikes, drove ISIL from the Syrian town of Kobane near Turkey.

Government forces and its allied fighters are waging their own campaign against ISIL, while also battling other groups including mainstream rebels and the al-Qaeda-linked Nusra Front in western areas that are mostly under state control.

'Australian killed'

An Australian who travelled to Syria to join the Kurdish forces was killed during the latest offensive in Hassakeh, the Observatory reported.

"An Australian man was killed in an assault on Tuesday by the Islamic State against a position of the Kurdish People's Protection Units [YPG] near Tal Hamis," said Abdelrahman.

"Dozens of Westerners have joined the YPG's ranks. There are foreigners fighting on all sides of Syria's war."

Source: Reuters