The Sunni extremist ISIS seized Tal Abyad from Kurdish and rebel combatants who have been fighting to oust the regime of Syria’s President Bashar al-Assad since 2011.

Syrian Kurdish forces have set their sights on taking back Tal Abyad, another strategic town on the border with Turkey, from ISIS militants, a monitor told Agence France-Presse Monday.

Tal Abyad, located about 65 kilometers east of Kobane, is a largely Kurdish town in the Syrian province of Raqqa and is being used by ISIS to cross into Turkey.

ISIS seized Tal Abyad from Kurdish and rebel combatants who had been fighting against Syria’s President Bashar al-Assad since 2011.

After four months of fierce fighting, Kurds and rebels recaptured Kobane in January, and they have since also reclaimed a third of the villages in the area, according to the Syrian Observatory for Human Rights.

“The next battle after Kobane is Tal Abyad,” Rami Abdel Rahman, the director of the Britain-based monitoring group, said on Monday.

“The Kurds and a Raqqa revolutionary brigade arrived on Monday at the edge of Raqqa province.”

An activist in Raqqa said the battle for villages around Tal Abyad had already begun, forcing people to flee across the border into Turkey.

“Tal Abyad is so important to ISIS that it has dug tunnels in the area and built fortifications on the town’s outskirts,” said the activist who identified himself as Nael Mustafa.

“The battle will take a long time, but it’s a start.”

On another front, further west in the province of Aleppo, “ISIS sent reinforcements to protect its strongholds of Minbej and Jarabulus which could also be a target for the Kurdish fighters,” said Abdel Rahman.

ISIS has taken advantage of the Syrian civil war and instability in Iraq to seize chunks of territory in the two countries, where it has committed atrocities that the U.N. has denounced as crimes against humanity.

A U.S.-led coalition which includes Arab and other Western countries, has targeted its positions with air strikes since September.

Meanwhile, Kurdish forces backed up by U.S.-led air strikes have recaptured territory from militants near the strategic city of Mosul in northern Iraq, the American military said Monday.

The advance is the latest push by Kurdish forces around Mosul, considered a crucial battleground for an eventual major counteroffensive against the ISIS group by Iraqi government troops and the Kurds.

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