The capture of Tel Abyad by the Kurdish YPG and smaller Syrian rebel groups means the Syrian Kurds effectively control some 400 km (250 miles) of the Syrian-Turkish border that has been a conduit for foreign fighters joining ISIS.

While the advance has brought the well-organized YPG deep into ISIS stronghold of Raqqa province, it has also concerned Turkey, which is worried the expansion of Kurdish sway risks inflaming separatist sentiment among its Kurdish minority.

Redur Xelil, the YPG spokesman, told Reuters that Tel Abyad was “under complete control.” The Syrian Observatory for Human Rights, a UK-based group that monitors the war, said most of the town was under YPG control with only a handful of ISIS fighters inside. Xelil said: “Most of them entered Turkey.”

The YPG has been the only significant partner to date in Syria for the U.S.-led alliance that is bombing ISIS. The YPG now controls the border from Syria’s far northeastern corner to just east of the town of Jarabulus. That is ISIS’s last remaining border crossing with Turkey, Xelil said.

The YPG-led forces also seized control of the road linking Tel Abyad to the ISIS’s de facto capital of Raqqa city, about 50 miles (80 km) to the south, cutting off a supply route which ISIS had used to send reinforcements.

Tel Abyad, on the border with Turkey, has been a main conduit for ISIS to smuggle weapons and oil.

The YPG-led forces had advanced into Raqqa after making big gains against ISIS in neighboring Hasaka province since early May, also with the help of the U.S.-led alliance.

While ISIS was being driven back in Hasaka, it was advancing elsewhere in Syria against government forces, notably in Palmyra, which the militants seized from the government in mid-May.

Turkish concerns

The fighting near the border has 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, he said.

Turkish President Tayyip Erdogan has voiced concern about the latest YPG-led offensive, saying Kurds were taking over areas from which Arabs and Turkmen were being displaced. He has accused the West of backing Kurdish “terrorists.” He has also said the outcome could eventually threaten Turkey’s border.

Turkey views the YPG as part of the PKK, which has fought a decades-long insurgency against Ankara and is considered a terrorist organization by Turkey, the United States and the European Union.

A Turkish official and humanitarian worker had said U.S.-led air strikes were partly to blame for the recent displacement of mainly Arab residents from the border area.

The U.S. Embassy in Ankara defended its strategy from accusations that it was hurting civilians, saying they were only targeting the militants and their activities.

In a statement published on Sunday the YPG urged civilians not to leave Syria, saying it was guaranteeing “their security and all of their humanitarian needs.”

The Syrian Observatory for Human Rights has said the people who fled did so as a result of the fighting rather than a systematic attempt by the YPG to force them out. It also denies Erdogan’s assertion that Turkmen were being forced out.

Turkey is already hosting 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 refugees have been fleeing more than four years of civil war which grew out of a March 2011 uprising against President Bashar al-Assad.