The US-backed Syrian Democratic Forces (SDF) have advanced against the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) near the armed group's stronghold in Raqqa in northern Syria, according to a monitor.

The SDF, an alliance of Kurdish and Arab fighters, now controls at least 40 percent of the town of Tabqa, and more than half of its heart, the Old City, the UK-based Syrian Observatory for Human Rights (SOHR) said on Sunday.

Rami Abdel Rahman, SOHR director, said fighting was continuing in the town on Sunday morning.

The SDF entered Tabqa on Monday as part of their offensive against Raqqa, ISIL's de-facto Syrian capital.

Supported by US-led coalition air strikes and special forces advisers, the SDF surrounded Tabqa in early April.

The town sits on a strategic supply route about 55km west of Raqqa, and served as an important ISIL command base, housing the group's main prison.

It is also adjacent to the Tabqa dam, another important strategic prize which remains under ISIL control.

"We gain more ground as we move towards the centre of the city," said SDF commander Mahdy Ayoob.

"We now control many neighbourhoods, and it is just a matter of time before Tabqa comes under our control."

Reporting from Gaziantep in neighbouring Turkey, Al Jazeera's Hashem Ahelbarra said the fighting in Tabqa "could be the most decisive moment" in the US-led military campaign against ISIL in Syria.

"The US-led coalition is intensifying its air strikes to break the group's defence," he said. "The Americans, who have expanded their presence in the area, hope the capture of Tabqa will speed up the defeat of ISIL."

Fierce resistance

The assault on Tabqa began in late March when SDF forces and their US-led coalition allies were airlifted behind ISIL lines.

Tabqa was home to around 240,000 residents before 2011, and more than 80,000 people have fled to the city from other parts of the country.

ISIL has put up fierce resistance, including using weaponised drones, a tactic the group honed in neighbouring Iraq.

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The group is also fighting street-to-street and using suicide attackers and car bombs to slow the SDF's advance, according to the SOHR.

The assault on Raqqa, dubbed Wrath of the Euphrates, was launched in November and has seen SDF fighters capture large expanses of countryside around the city.

More than 320,000 people have been killed in Syria since the country's war began with anti-government protests in March 2011.