Islamic State has claimed responsibility for an attack that seven guards in the northern Syrian city of Manbij, days after its “caliphate” was declared defeated.

Manbij is a former ISIS stronghold that is now ruled by the Manbij Military Council affiliated with the Syrian Democratic Forces, the U.S.-backed alliance which declared victory over ISIS in its last redoubt in eastern Syria on Saturday.

At around midnight (2200 GMT) on Monday, March 26, gunmen opened fire at fighters manning a checkpoint at the entrance to the city, killing seven, the Manbij Military Council said on Tuesday.

#عاجل

تم تنفيذ هجوم إرهابي على إحدى الحواجز في مدخل مدينة منبج، و ذلك في منتصف الليل حوالي الساعة 12 ليلة أمس و عل أثره تم استشهاد سبعة مقاتلين كانوا يقومون بمهامهم لحراسة المدينة. المركز الإعلامي لمجلس منبج العسكري — المجلس العسكري لمنبج (@mmc2016c) March 26, 2019

“The soldiers of the caliphate attacked a checkpoint … west of the city (Manbij) last night,” said a statement published on ISIS social media channels.

MMC spokesperson Sherfan Darwish said earlier Tuesday that it could be a revenge attack by ISIS sleeper cells.

“After the victory over IS, we have entered the phase of sleeper cells,” Darwish told AFP.

“These sleeper cells are being activated and carrying out attacks but we will foil their operations.”

ISIS has vowed to carry out revenge attacks against the SDF for the six-month offensive which culminated in the jihadists’ defeat in the village of Baghuz, close to the Iraqi border, on Saturday.

Manbij is also a major point of contention between the Kurds, who form the backbone of the SDF, and neighboring Turkey, which is deeply opposed to their autonomous administration in northeastern and parts of northern Syria.

The city is one of the few areas west of the Euphrates that remains under Kurdish influence after Turkish troops and their Syrian rebel allies overran the Kurdish enclave of Efrin in March last year.

ISIS claimed responsibility for a January suicide attack in Manbij that killed almost a dozen people, including U.S. military personnel.

In December, Ankara threatened to launch a new offensive to dislodge the People’s Protection Units (YPG) – the core component of the SDF – from the entire length of the border.

Turkey considers the YPG a terrorist organization due to its ties to the Kurdistan Workers’ Party, or PKK, which has fought an insurgency in the country since the 1980s.

With reporting from AFP