A Kurdish-Syrian force is advancing toward one of the Islamic State’s most strategically vital possessions, capturing territory in the group’s landmark province of Raqqa and threatening to inflict what could be the most significant defeat yet for the militants.

The Kurdish-led force, backed by U.S. airstrikes, closed in from the south, east and west on Saturday on the Syrian-Turkish border town of Tel Abyad, a key Islamic State stronghold on which the militants rely for trade with the outside world and also the flow of foreign fighters who sustain their strength on the battlefield.

The Kurdish militias and their allies are within six miles of the town and could soon be in a position to encircle it, isolating the Islamic State’s self-proclaimed capital in the city of Raqqa farther south, according to statements from the People’s Protection Units, or YPG, the main Kurdish fighting force.

The advance is putting the Islamic State on the defensive only weeks after the group celebrated victories in the Iraqi city of Ramadi and the Syrian city of Palmyra, after the Iraqi and Syrian armies crumbled in their respective battles.

The progress demonstrates that success is possible when a well-motivated and coordinated force is backed by U.S. airstrikes, said Abu Shujaa, a spokesman for Thuwar al-Raqqa, or Raqqa Revolutionaries, one of the Syrian rebel battalions fighting in the coalition force.

Fighters from the Kurdish People’s Protection Units gather belongings of Islamic State fighters in the town of al-Mabroukah after taking control of the area May 28, 2015. (Rodi Said/Reuters)

“Daesh is not as strong as it thinks, but its enemies are weak. We are successful because we have the will to fight,” Abu Shujaa said, referring to the Islamic State by its Arabic acronym.

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

The offensive raises the specter of another major battle on the Turkish border similar to the one that dominated headlines last fall for the much smaller Kurdish town of Kobane — except that, in this case, the Islamic State would be the force defending a town.

The offensive to capture Tel Abyad is effectively a continuation of the Kobane battle, which was a turning point for the Islamic State’s expansion in northeastern Syria. The Islamic State had been poised to capture the Kurdish town until the United States intervened with airstrikes and halted the militants’ advance.

Since then, the Kurdish YPG has reversed the tide of the fight, steadily pushing the Islamic State back to the point where the militants are losing territory they had held for more than 18 months in their heartland in the province of Raqqa.

No longer can the Islamic State claim control of an entire province in either Syria or Iraq.

At Friday prayers in the city of Raqqa, imams urged citizens to stockpile supplies of flour and food in preparation for a potential siege, according to the activist group Raqqa Is Being Slaughtered Silently, which reports from inside the city.

A week earlier, the group reported, the Islamic State evacuated the families of foreign fighters from Tel Abyad, relocating them to Raqqa.

The Kurds who dominated the battle in Kobane have been joined by several Free Syrian Army units. They are fighting as a coalition called Burkan al-Furat, or Euphrates Volcano. Forces with the coalition also have advanced from Kurdish-held territory to the east of Tel Abyad. On Saturday, they encircled the town of Suluk, to the south of Tel Abyad, further pressuring the Islamic State.

The participation of Arab rebels from the Free Syrian Army is important because most of the population of Raqqa province, including Tel Abyad, is Arab, said Aras Xani, a fighter with the Kurdish YPG on the eastern front of the battle.

“More and more Free Syrian Army fighters are taking part because the population of this area is mostly Arab. Arabs and the FSA must play a big role in this operation since it is their homeland,” he said.

Abu Mohanned, a commander with the Free Syrian Army units advancing from the west toward Tel Abyad, said Islamic State fighters had retreated without a fight from many of the villages his forces have taken in their advance on Tel Abyad. “When we meet resistance, we send the coordinates to the coalition, and they carry out airstrikes,” he said.

The Islamic State is expected to put up a much tougher fight for Tel Abyad, given its importance. The town adjoins Turkey, and although the official border crossing has been closed since the town fell under Islamic State control a year ago, smuggling routes nearby serve as the group’s lifeline to the outside world.

“Resistance is growing the closer we get to Tel Abyad,” said Xani, who predicted another month of fighting before the town falls.