U.S.-backed Kurdish fighters took control of a strategic town on the Syria-Turkey border Monday, forcing ISIS militants to flee and cutting off a key supply line to the self-proclaimed caliphate's capital.

The Washington Post reported that the main Kurdish fighting force, known as the YPG, backed by affiliated Syrian rebels, had captured the town of Tal Abyad, claiming control of the town center by nightfall Monday. The Post also reported that the advancing forces had cut off ISIS' escape route from the town, surrounding it from the east, south, and west.

The loss of Tal Abyad, some 50 miles north of Raqqa, the capital of ISIS' self-declared caliphate, is the extremists' biggest setback since Kurdish fighters took control of the border town of Kobani near Turkey, after fighting ISIS for months. The Kurdish victory deprives ISIS of a direct route for bringing in foreign militants and supplies, and links the Kurds' two fronts, putting even more pressure on Raqqa.

An anti-ISIS media collective based in Raqqa said the extremists had set up checkpoints in the center of the city on Monday and installed security cameras in a main square.

The Britain-based Syrian Observatory for Human Rights confirmed to the Associated Press the that Kurdish fighters had "almost full control" of Tal Abyad by Monday evening, and had taken command of the border crossing with Turkey. It said some 40 Islamic State militants were targeted by U.S.-led airstrikes as they tried to flee south.

An AP photographer in Akcakale, on the Turkish side of the border, saw several dozen YPG fighters waving their yellow triangular flag and flashing victory signs. Earlier, several dozen Kurdish gunmen were seen running up a hill, moving west.

A few people on the Syrian side of the border were seen raising the green, white and red flag of the Free Syrian Army before being apprehended by Turkish security after they broke a hole in the border fence. A contingent of Free Syrian Army fighters is battling alongside the Kurds in an effective alliance against ISIS called "Burkan al-Furat," or Volcano of the Euphrates.

Earlier, Kurdish units marching west from Kobani and others marching east from the Kurdish town of Ras al-Ayn met up in the village of Qaysariyeh, some two miles south of Tal Abyad as they encircled the town from three sides, leaving Turkey as the only outlet.

As with the Kurdish victory in Kobani, the YPG fighters' advance under the cover of the U.S-led air campaign highlighted the decisive importance of combining airstrikes with the presence of a cohesive and motivated ally on the ground — so clearly absent in Iraq and other parts of Syria.

With most of Syria now controlled by either ISIS or forces loyal to President Bashar Assad, the U.S. has found a reliable partner in the YPG, a group of moderate, mostly secular Kurdish militiamen driven by revolutionary fervor and the desire for self-rule.

Since the beginning of the year, they have wrested back more than 500 mostly Kurdish and Christian towns in northeastern Syria, as well as strategic mountains seized earlier by the Islamic State group. They have recently pushed into Raqqa province, an ISIS stronghold where Tal Abyad is located.

The Kurdish advance has caused the displacement of more than 16,000 people who fled to Turkey in the past two weeks. On Monday, up to 3,000 more refugees arrived at the Akcakale border crossing, according to Turkish state-run TRT television. An AP photographer saw large numbers of people at the border and thick smoke billowing as U.S.-led coalition aircraft targeted IS militants in Tal Abyad.

As Kurdish fighters push deeper into ISIS strongholds in northern Syria, tensions with ethnic Arabs and Turkmen in the region have risen.

On Monday, more than a dozen Syrian rebel groups accused the Kurdish fighters of deliberately displacing thousands of Arabs and Turkmen from Tal Abyad and the western countryside of predominantly Kurdish Hassakeh province. In a statement, they accused the YPG of committing "ethnic cleansing" — a charge strongly denied by the Kurds.

The accusation, which was not backed by evidence of ethnic or sectarian killings, threatened to escalate tensions between ethnic Arabs and Kurds as the Kurdish fighters conquer more territory in northern Syria.

"YPG forces ... have implemented a new sectarian and ethnic cleansing campaign against Sunni Arabs and Turkmen under the cover of coalition airstrikes which have contributed bombardment, terrorizing civilians and forcing them to flee their villages," the statement issued by rebel and militant groups said.

The 15 rebel groups, including the powerful ultraconservative Ahrar al-Sham and Jaish al-Islam, said the alleged ethnic cleansing was concentrated in Hassakeh province and in Tal Abyad, and was part of a plan by the Kurdish Democratic Party, or PYD, to partition Syria. The YPG, or People's Protection Units, is the armed wing of the PYD. The movement is affiliated with the Kurdish PKK, which has waged a long and bloody insurgency in southeastern Turkey.

The statement echoed comments last week by Turkish President Recep Tayyip Erdogan.

"On our border, in Tal Abyad, the West, which is conducting aerial bombings against Arabs and Turkmen, is unfortunately positioning terrorist members of the PYD and PKK in their place," Erdogan said.

Khalil, the YPG spokesman, strongly refuted the claim, and seeking to calm nerves, said the YPG is a Syrian national group whose battles are directed solely against ISIS.

"We say to residents of Tal Abyad, there is no reason for you to cross to another country (Turkey). Our towns are open to you, you are our people and you will return to your towns, villages and properties," he said.

He pledged that the YPG will not interfere in administering Tal Abyad once it falls, leaving it to civilian committees.

The Associated Press contributed to this report.

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