ISIS is facing one of its most serious setbacks yet

ISIS is currently facing one of its greatest military challenges since the group proclaimed a caliphate following the seizure of Mosul and much of western Iraq and eastern Syria last summer.

According to an intelligence brief from The Soufan Group, ISIS is experiencing losses around its “capital” of Raqqa, representing both an operational and symbolic setback for the group.

Although ISIS has continued to expand and hold onto territory in Iraq, the militants have come under increasing pressure in Syria.

ISIS has lost territory in a number of key battles. Most notably, Kurdish YPG forces have dealt ISIS defeats at the towns of Tal Abyad and Ayn Issa.

ISIS once hoped to cut off major Syrian Kurdish regions from one another by holding these towns near the Turkish border. Now, the Kurds have foreclosed on that strategy, beating back the jihadists’ momentum and even moving into some of ISIS’s most important territory.

“With the most recent YPG moves against the town of Ayn Issa, the Islamic State is facing perhaps its most serious symbolic and meaningful threat since it declared itself a caliphate almost one year ago,” The Soufan Group notes. “Its capital, Raqqa, the center of the group’s authority and image, is under threat.”

By seizing and securing Ayn Issa, the YPG, in conjunction with US-led coalition airstrikes, have embedded themselves only 31 miles away from ISIS’ de facto capital. The YPG also seized the Syrian military base Liwa-93 from ISIS in the surrounding region. The rapid advance of the Kurdish forces, which ISIS nearly overwhelmed during a crucial battle in the border city of Kobane last summer, has dealt a blow to the militant group, which promoted itself through a doctrine of “remaining and expanding” on multiple simultaneous battlefronts.

Following the YPG’s gains, ISIS forces began digging trenches around Raqqa in an attempt to fortify their capital, Reuters reports. ISIS spokesman Abu Muhammad al-Adnani also addressed the losses in a Ramadan audio broadcast stating that “God never gave the mujahideen a promise of victory every time.”

Although the Kurds do not have immediate plans to attack ISIS in Raqqa, the seizure of territory around the city could deal a significant blow to the militant organization. Tal Abyad, located by the Turkish border, functioned as a key smuggling point through which fighters and supplies could reach the jihadists.

With its opponents taking control of the territory north of Raqqa, ISIS could experience significant logistical disruptions — and face the crisis of enemy forces advancing closer to the heart of the group’s power.

Speaking to The Wall Street Journal, Edgar Vasquez, a spokesperson for the US State Department’s Near Eastern Affairs Bureau, said that “should anti-ISIL forces continue to hold the city, there is the potential for a significant disruption of ISIL’s flow of foreign fighters, illicit goods, and other illegal activity from Turkey into northern Syria and Iraq.”

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This article originally appeared at Business Insider Defense. Copyright 2015. Follow BI Defense on Twitter.

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