7 US-backed fighters in Syria killed in Manbij sleeper cell attack

 In this March 28, 2018 file photo, a fighter guards the entrance of the Manbij Military Council, the Kurdish-led, US-backed, militia group that defends Manbij, Syria. AP

A suspected ISIS sleeper cell attack killed seven US-backed fighters in Syria’s Manbij, according to the northern city’s military council, days after the Syrian Democratic Forces declared the defeat of the ISIS “caliphate”.

Once an ISIS stronghold, Manbij is now ruled by a military council affiliated with the SDF, the Kurdish-led group that announced victory over ISIS after clearing its last pocket of territory in eastern Syria on Saturday.

Overnight on Monday, unidentified gunmen attacked fighters manning a checkpoint on the outskirts of Manbij, the council said.

There was no immediate claim of responsibility but council spokesman Sherfan Darwish said he suspected an ISIS sleeper cell.

"After the victory over ISIS, we have entered the phase of sleeper cells," Mr Darwish told AFP.

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

The SDF has been warning of the likelihood of such attacks in the wake of their victory announcement.

SDF commander-in-chief Mazlum Kobane wrote in The Defense Post on Monday: "Though the structure of ISIS will come to an end, we also want to draw attention to some major challenges that are ahead of us: sleeper cells planted by the terrorist organisation, and the danger in ISIS's ability to reorganise itself by employing tactics of individual terrorist acts such as bombings and assassinations."

The attack was the deadliest in Manbij since the January 16 suicide bombing on a restaurant that killed 19 people, including four US service personnel. That attack was claimed by ISIS.

The Syrian Observatory for Human Rights, a Britain-based war monitor, agreed that the attack had the hallmarks of an ISIS cell, which would make it "the first attack of its kind" since the SDF’s victory announcement.

ISIS has vowed to continue its struggle in the wake of a six-month offensive by the SDF that culminated in a final declaration of victory on Saturday in the village of Baghouz near the Iraqi border.

In an audio message relased last week, ISIS spokesman Abu Hassan Al Muhajir exhorted surviving followers to "avenge the blood of your brothers and sisters... Set up the [explosive] devices, deploy the snipers."

Hundreds of SDF fighters have been killed in sleeper cell attacks since August, according to the Observatory.

The SDF says it lost more than 11,000 fighters killed and a further 12,000 injured in its war against ISIS.