Kurdish forces are engaged in a major battle to retake the city of Sinjar in northern Iraq from Islamic State (IS) forces.

British and American special forces, as well as US-led airstrikes, are supporting the operation on the front line.

US-led coalition airstrikes have been pounding IS-held areas in the town - directed by US and British special forces on the frontline, according to Kurdish Peshmerga commanders, Sky News reports.

The bombardment, which has killed 60-70 IS militants, has paved the way for around 7,500 Kurdish special forces, Peshmerga and Yazidi fighters to descend from Sinjar mountain towards the frontline.

They travelled in a convoy made up of humvees on flatbed trucks, heavy artillery and fighters waving Kurdish flags and brandishing their rifles - winding past abandoned cars and bloodstained clothing on the road many of them had used to flee IS in 2014.

On the frontline, walkie-talkie chatter has picked up militants urging each other to fight to the death and an order given by the local IS emir that anyone "withdrawing from the caliphate" should be shot by their own side.

Operation Free Sinjar aims to cordon off the town, which sits on the main highway between Mosul and Raqqa - the main IS bastions in Iraq and Syria.

Kurdish forces claim they have captured part of Sinjar's main road and started clearing the town itself since the offensive launched.

Sinjar was overrun more than a year ago by IS, who massacred and enslaved thousands of Yazidis - regarded as devil worshippers by the extremists.

The onslaught prompted Barack Obama to authorise the first airstrikes against Islamic State in August 2014, saying he was acting to prevent a genocide of the Yazidis.

Hussein Derbo, the head of a Peshmerga battalion made up of 440 Yazidis, said the men under his command could have migrated to Europe but chose to stay and fight.

"It is our land and our honour. They (Islamic State) stole our dignity. We want to get it back," he told Reuters in a village on the northern outskirts of Sinjar town.

Derbo's brother, Farman, echoed the sentiment, saying he hoped the militants did not retreat from battle so that the Yazidis could kill them all.

The offensive is being personally overseen by Kurdistan regional president Massoud Barzani.

Kurdish forces and the US military said the number of IS fighters in the town had increased to nearly 600 after reinforcements arrived as the offensive was delayed by weather and friction between various Kurdish and Yazidi forces.