The self-proclaimed Islamic State in Iraq and Syria (ISIS) has launched an assault on the largely Kurdish city of Hasakah in northeast Syria on Saturday, the Syrian Observatory for Human Rights said.
The attack came a day after Kurdish militia executed 20 civilians accused of supporting the extremists, the monitoring group said.
"ISIS fighters started an assault on Hasakah city and reached the city's southern periphery, where they are clashing with pro-regime forces," said Observatory head Rami Abdel Rahman.
He said the pro-government National Defence Forces militia had blocked the IS advance on the city's edges and reported heavy shelling.
Syria's official news agency SANA said loyalist forces had "foiled a terrorist attack by IS on military points near Hasakah".
Control of Hasakah, capital city of the province by the same name, is split between President Bashar al-Assad's regime and Kurdish forces.
Abdel Rahman said at least 10 government loyalists had been killed, but he had no immediate information on ISIS casualties.
The Assyrian Network for Human Rights activist group said IS had launched "a new attempt at seizing control of the city" and seized one checkpoint on Hasakah's southwest edges.
"Hasakah has been hit by mortar rounds since Saturday morning," and a local church had been damaged, the Network said.
Elsewhere in Hasakah province, Kurdish militia executed at least 20 civilians Friday, including two children, after accusing them of being ISIS supporters, the Observatory reported.
The Britain-based monitor said Kurdish militia also burned and demolished homes of people it suspected of supporting IS near the flashpoint towns of Ras al-Ain and Tal Tamr.
ISIS on Friday killed 30 civilians trying to escape an advancing assault, Abdel Rahman said.