Beirut (AFP) - Kurdish militia have expanded their control over portions of a major Syrian city in their fightback against the Islamic State group, to the detriment of government forces there, a monitor said Saturday.

"The Kurdish People's Protection Units (YPG) are advancing in Hasakeh city against IS and at the expense of the regime," said Rami Abdel Rahman, head of the Syrian Observatory for Human Rights.

The YPG and government forces have both been defending the city from an IS assault, which began last month.

The Britain-based Observatory said the YPG now controls a majority of the metropolis, despite Kurds making up just under a third of its population.

"The YPG control 70 percent of Hasakeh city, IS controls 10 percent and the regime controls 20 percent," Abdel Rahman told AFP.

Before the IS began its offensive by seizing territory in southern neighbourhoods from regime forces, the Kurdish militia controlled less than half of Hasakeh.

When the YPG pushed the jihadists out of some of these areas, they maintained control of them.

The YPG also directly expanded into areas held by loyalists in the city centre, although it did not engage in clashes with them.

The militia now hold territory in Hasakeh north, west, centre and south, said Abdel Rahman, with IS left "surrounded" in four small neighbourhoods in the south.

More than 230,000 people have been killed in Syria since anti-government protests erupted in March 2011 before degenerating into civil war.