Sporadic clashes between the Islamic Sate of Iraq and the Levant (ISIL) and US-backed fighters have erupted in the northern ISIL-held Syrian town of Manbij after the group ignored a 48-hour offer to withdraw from the besieged town without a fight, opposition activists and a Kurdish official said.

The UK-based Syrian Observatory for Human Rights said about 200 civilians fled the town on Friday. A 20-year-old woman among those fleeing died when she stepped on a land mine while trying to escape with her children, the Observatory said.

Members of the predominantly Kurdish US-backed Syria Democratic Forces (SDF) have been on the offensive in Manbij for weeks, backed by coalition airstrikes.

On Thursday, the Manbij Military Council - part of the SDF - said ISIL fighters were given 48 hours to leave the town with their "individual weapons," saying this was their last opportunity to leave alive.

READ MORE: ISIL given '48 hours' to leave Manbij

Sherfan Darwish of the SDF said the group did not respond to the offer and that sporadic clashes erupted on Friday. The ISIL-affiliated Amaq news agency said the US-led coalition carried about 20 airstrikes on the center of Manbij on Friday.

Manbij lies on a key supply route to the group's de facto capital of Raqqa. If the town is captured by the US-backed fighters it would be the "biggest strategic defeat" for ISIL in Syria since July 2015, when the group lost the border town of Tal Abyad, said Al Jazeera's Bernard Smith, reporting from Turkey-Syria border.

'Human shields'

Meanwhile, US-led coalition spokesman Colonel Chris Garver said ISIL "used civilians as human shields and as bait" in an effort to draw the fire of the SDF toward civilians.

Garver's comment comes after US-led coalition air strikes allegedly killed 56 civilians, including 11 children, as they fled on Tuesday from a village near Manbij.

Garver said the attack on Tuesday came after SDF fighters "observed a large group of Daesh [ISIL] fighters in a convoy who appeared to be readying for a counterattack" against US-backed troops in the area.

"A strike was called in on Daesh. The strike was against both buildings and vehicles." Afterwards, the spokesman said, the coalition received both internal and external reports "that there may have been civilians in the area who are mixed in and among the Daesh fighters."

Garver said the first phase of the investigation - what he called a "credibility assessment" - would take no longer than a week and a half.

In Geneva, spokesman Jens Laerke of the UN Office for the Coordination of Humanitarian Affairs said a convoy carrying assistance for 32,000 people arrived Friday in the hard-to-reach town of Halat al-Madeh in the central province of Hama.

"This is the first inter-agency convoy to Hama in 2016," said Laerke, adding that the Syrian government had removed some "surgical and certain medical items" from the cargo.

The United Nations says there are nearly half a million people in besieged areas in Syria and an estimated 4.5 million Syrians in so-called hard-to-reach areas.