The Kurdish People’s Protection Units — with U.S.-led air support — took complete control of the Liwa (Brigade) 93 base on Monday, the Syrian Observatory for Human Rights said.

“[ISIS] lines of defense were pushed back to the gates of Raqqa,” added Rami Abdel Rahman, director of the Observatory.

“The Kurds also entered the outskirts of the town of Ayn Issa, close to Brigade 93… this town has basically fallen in the military sense with the capture of the base.”

The base was important due to its position overlooking strategic roads linking Raqqa to other ISIS outposts in the provinces of Aleppo to the west and Hasakah to the east.

ISIS militants had held the military base after capturing it from regime forces last summer.

The taking of the base is the second blow inflicted on ISIS fighters by the Kurds in a week after the capture of Tal Abyad, on Syria’s border with Turkey.

Tal Abyad was a key conduit for foreign fighters and supplies into ISIS -held territory in Syria and for exports of black market oil from jihadist-held fields.

Kurdish forces defended the Syrian town of Kobane against ISIS in a months-long battle, and have emerged as some of the most effective forces battling the jihadist group in the year since it declared a cross-border “caliphate” with neighboring Iraq.