The Syrian Kurdish YPG militia said on Monday, June 15 it had encircled the Islamic State-controlled town of Tel Abyad, the nearest border town to the militant’s de facto capital of Raqqa city, according to Reuters.

YPG spokesman Redur Xelil said the militia had surrounded the town along the Turkish border, pushing ahead with an offensive with the help of U.S.-led airstrikes to seize strategic territory held by the jihadists, including their main Tel Abyad-Raqqa supply route.

“Tel Abyad is almost besieged now after the control of the Raqqa-Tel Abyad road,” he said.

The Kurdish units were sending reinforcements to the area south of Tel Abyad from both its stronghold in Hasaka province in the northeastern border area and from Kobani, northwest of Tal Abyad.

Fighting near the border has already forced more than 18,000 people to cross into Turkey from Syria, aid workers say. A further 5,000 are believed to have crossed on Monday, according to a Reuters photographer at the scene.

The YPG has emerged as the main partner on the ground in Syria for the U.S.-led alliance that has been bombing Islamic State in Syria and Iraq. Its advance into Raqqa province follows a campaign that drove Islamic State from wide areas of neighboring Hasaka province

For the YPG, seizing Tel Abyad would help them link up Kurdish-controlled areas in Hasaka province and Kobani.

The expansion of Kurdish influence in Syria near the border with Turkey is a concern for Ankara, which has long been worried about separatism among its own Kurdish population.