A Kurdish-Syrian force advanced on a key town on the Turkish-Syrian border held by the Islamic State group Saturday, sending hundreds of refugees fleeing and raising the prospect of cutting off ISIS' capital city Raqqa's connection to Turkey.

A statement from the main Syrian Kurdish fighting force, cited by the Associated Press, said its fighters have encircled the ISIS-held town of Suluk. Fighters are now reportedly advancing on the nearby strategically important town of Tal Abyad.

Tel Abyad is highly significant for ISIS, which uses the town's proximity to Turkey to facilitate the flow of crucial supplies and foreign fighters to its capital, Raqqa, to the south.

“[ISIS] is not as strong as it thinks, but its enemies are weak. We are successful because we have the will to fight,” Abu Shujaa a spokesman for Thuwar al-Raqqa, or Raqqa Revolutionaries, one of the Syrian rebel battalions fighting in the coalition force, told the Washington Post.

“And of course, we are getting help from the coalition in the form of airstrikes,” he added.

Rami Abdel Rahman, head of the British-based Syrian Observatory for Human Rights, said there were only 150 ISIS fighters holding Tal Abyad itself, and that they had threatened to withdraw if they did not receive reinforcements from Raqqa.

"But the leadership in Raqqa will not send them reinforcements, because the coalition air strikes have been decimating [ISIS]," Mr Abdel Rahman said, according to agency reports.

A video shared on social media, the authenticity of which has yet to be confirmed, appeared to show hundreds of refugees who had fled the fighting, gathered at a Turkish border crossing.

Some 13,000 refugees have crossed into Turkey in the past 10 days, according to a Turkish Foreign Ministry statement, cited by the Associated Press.

The expected assault on ISIS-held territory is a significant military reversal for the group, which recently celebrated victories in the Iraqi city of Ramadi and the Syrian city of Palmyra.

A map shared on social media gives an overview of the current situation on the ground, with the areas marked YPG representing the Kurdish People's Protection Brigades and their coalition partners.