Beirut and Akçakale, Reuters—The Syrian–Kurdish People’s Protection Units (YPG) militia said on Monday it had seized a major road that brought reinforcements from the Islamic State of Iraq and Syria’s (ISIS) de facto capital of Raqqa, enabling YPG fighters to lay siege to the extremist group’s stronghold of Tel Abyad.

YPG spokesman Redur Xelil said the militia had surrounded the town along the Turkish border, pushing ahead with an offensive with the help of US-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 Al-Hasakah province in the northeastern border area and from Kobani, northwest of Tel Abyad.

The Kurdish official said control of the road will prevent any reinforcements by the militants from Raqqa further south.

The loss of Tel Abyad would leave ISIS with only the Jarablus border crossing along the Turkish border in their hands.

Tel Abyad has been a main conduit for weapons and smuggling of oil by the militants through Turkey.

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.

Soldiers directed the people, many of whom were elderly, women and children, through a passage in a barbed wire fence to a border facility.

A Turkish official and humanitarian worker had said US-led airstrikes were partly to blame for the recent displacement of mainly Arabs inhabiting the border area. The US Embassy in Ankara defended its strategy from accusations that it was hurting civilians, saying they were only targeting the militants and their activities.

Turkey is already hosting some 1.8 million Syrians, more than any of Syria’s other neighbors and one of the biggest refugee populations in a single country anywhere in the world.

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

Veysel Ayhan, president of International Middle East Peace Research (IMPR), a think-tank which has an office of its humanitarian arm in Akçakale, said the YPG along with Syrian opposition forces were very close to taking the town.

The coalition airstrikes had prevented ISIS from sending additional fighters from its Raqqa stronghold, he added.

For the YPG, seizing Tel Abyad would help them link up Kurdish-controlled areas in Al-Hasakah 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.