ISIS has been forced into retreat across parts of the strategic region, a land bridge between territory it controls in Syria and Iraq, even as its fighters have mounted new raids this week on Assyrian Christian villages, abducting more than 200 people.

"The flag is flying over Tel Hamis. We are now combing the city for terrorists and mines," militia spokesman Redur Xelil told Reuters.

The British-based Observatory said Kurdish forces killed at least 175 members and commanders of the ultra-hardline Islamist militants in an offensive which began last weekend.

"The bodies of these terrorists are still with the Kurdish militants," said the Observatory's head Rami Abdulrahman. He said the Kurds have taken at least 103 villages in the area and are now in the village of Suleima on the border with Iraq.

Kurdish forces dealt a blow to ISIS by capturing an important town on Friday in the latest stage of a powerful offensive in northeast Syria.