Arab and Kurdish forces have killed at least 16 fighters from the ISIS militants group during fierce clashes north of the militants' stronghold of Raqa, a monitor said Sunday.

ISIS launched an offensive last Wednesday against areas held by the Syrian Democratic Forces coalition near to Ain Issa, a town held by the SDF some 50 kilometres (30 miles) from Raqa, killing 21 Kurdish fighters.

Late on Saturday, "16 ISIS were killed and 19 wounded" in clashes with the SDF near Ain Issa, said Rami Abdel Rahman of the Syrian Observatory for Human Rights.

The SDF also recaptured an area seized by IS a few days earlier, he said.

The alliance, made up of units from the Kurdish People's Protection Units (YPG) and Arab fighters, has carried out several major operations against IS.

On December 26, it seized the Tishreen Dam from IS along with several villages on the Euphrates' eastern bank.

The dam, held by IS since 2014, helps generate electricity for large parts of the northern Aleppo province, the Observatory said.

It was the alliance's second major operation, after clearing IS from some 200 villages in the northeastern province of Hasakeh.

On the Euphrates' western bank, IS still controls swathes of territory stretching from Raqa to Jarablus, on the border with Turkey.

More than 260,000 people have been killed in Syria since March 2011.

The conflict started as anti-government protests, but spiralled into a complicated multi-sided conflict.