Sporadic clashes erupted between Islamic State militants and US-backed fighters on Friday in a northern, IS-held Syrian town, after the extremists ignored an offer to leave without a fight.

The fighting is forcing many civilians to flee, and the Britain-based Syrian Observatory for Human Rights said about 200 civilians fled the town of Manbij in the morning.

A 20-year-old woman who was among those fleeing died when she stepped on a land mine while trying to escape with her children, the Observatory said.

A Facebook page that covers Manbij posted two photographs of a few dozen people, mostly women and children, saying they “risked their lives” to flee the town’s southern district of Hazawneh.

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

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

Sherfan Darwish of the SDF said the extremists did not respond to the offer and that fresh clashes erupted on Friday. The IS-linked Aamaq news agency said the US-led coalition carried out about 20 air strikes on the centre of Manbij on Friday.

Manbij is an IS hub and lies on a key supply route to the group’s de facto capital of Raqqa. If Manbij is captured by the US-backed fighters, it will be the biggest strategic defeat for IS in Syria since July 2015, when the extremist group lost the border town of Tal Abyad.

In Geneva, spokesman Jens Laerke of the United Nations Office for the Coordination of Humanitarian Affairs said a convoy carrying assistance for 32,000 people had arrived 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 Mr Laerke, although he added the Syrian government had removed some “surgical and certain medical items” from the cargo.

The UN said 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.

In Moscow, Russia’s Foreign Minister Sergey Lavrov said US Secretary of State John Kerry had reiterated the West’s stance that Syrian President Bashar Assad must leave power, during a recent visit to Russia.

Moscow, a major Assad ally, disagrees with that, fearing worse turmoil if the Syrian president were to go.

Mr Lavrov said: “Kerry… said we need to agree first that Assad goes. Who can guarantee that what happened to Libya won’t happen to Syria?”