Advancing Kurdish and Arab fighters backed by US-led air strikes now control 40 percent of the Islamic State stronghold of Manbij in northern Syria, a monitor said Sunday.

The Syrian Observatory for Human Rights said the Syrian Democratic Forces (SDF) had pushed deeper into the town near the border with Turkey, with air cover from the US-led coalition against the jihadists.

Around 2,300 civilians have fled Manbij in the past 24 hours as the SDF fighters advanced, according to the Britain-based monitor.

It said clashes between the joint Kurdish-Arab force and IS fighters were continuing in several parts of the town.

"It's a street battle, and the process of eating away at IS territory is ongoing," Observatory director Rami Abdel Rahman told AFP.

He said the SDF had advanced into eastern parts of Manbij, which is located in Aleppo province on IS's main supply route between Syria and Turkey.

The SDF began its offensive to retake Manbij from IS on May 31, but progress slowed after it entered the town because of a fierce counteroffensive by the jihadists.

Thousands of civilians have already fled but thousands more are believed to remain, and there have been concerns about their fate as heavy fighting continues.

Earlier in the month, the SDF gave IS an ultimatum to leave Manbij within 48 hours, offering to allow fighters to flee with light weapons in what it described as a bid to protect civilians.

The initiative came after at least 56 civilians, including children, were reportedly killed in US-led air strikes near Manbij.

The coalition has said it is investigating the deaths, which provoked a sharp backlash, including a call from the Syrian opposition National Coalition for the US-led strikes to be suspended.

The 48-hour ultimatum was ignored by IS and fighting for the town has continued.

More than 280,000 people have been killed in Syria since the conflict began in March 2011 with anti-government protests.

The conflict has evolved into a complex multi-front war that has displaced over half Syria's population.

Source: AFP