Activists say Islamic State militants have attacked a string of predominantly Christian villages in northeastern Syria, touching off heavy clashes with Kurdish militiamen and their local allies.

The Assyrian Network for Human Rights and the Britain-based Syrian Observatory for Human Rights say the Islamic State group launched its assault against the villages near the town of Tal Tamr along the Khabur River around dawn on Saturday.

The militants took more than 220 Christian Assyrians captive from villages in the same area last month.

Assyrian Network for Human Rights director Osama Edwards says the "battles are now very intense, very violent."

Observatory director Rami Abdurrahman says the Islamic State extremists initially made gains before Kurdish fighters and local militiamen pushed them back.

There was no immediate word on casualties.