A quarter of Raqqa has been seized from ISIS

US-backed fighters have seized a quarter of Syria's Raqqa from ISIS group, a monitor said Monday, less than three weeks after they first entered the northern city.

Arab and Kurdish militiamen from the Syrian Democratic Forces smashed into the militants’ main Syrian bastion on June 6 after a months-long drive to encircle it.

"Since the offensive began, the SDF have captured around 25 percent of the city's built-up neighborhoods," Rami Abdel Rahman, head of the Syrian Observatory for Human Rights, told AFP on Monday.

Backed by US-led coalition air strikes, the SDF has fully seized the southeastern districts of Al-Meshleb and Al-Senaa, as well as Al-Rumaniya and Sabahiya in the west, he said.

From those neighborhoods, they were bearing down on Raqqa’s Old City in a pincer movement on Monday, with fighting raging in the western Al-Qadisiya district and parts of the city's east.

SDF fighters also hold part of Division 17 -- a former Syrian army base -- and an adjacent sugar factory on the northern edges of the city.

"They want to cut off the city's northern part, including the Division 17 base, so that there's more pressure on IS in the city center," Abdel Rahman said.

The battle for Raqqa is the SDF's flagship offensive, with heavy backing from coalition air strikes, advisers, weapons and equipment.

The US-led coalition is also backing a major assault on the last ISIS-held pockets of Mosul in neighboring Iraq.

ISIS overran Raqqa in 2014, transforming it into the de facto Syrian capital of its self-declared "caliphate".

It became infamous as the scene of some of the group's worst atrocities, including public beheadings, and is thought to have been a hub for planning attacks overseas.