The Islamic State (ISIS)  killed at least 128 people in a Syrian town deep inside territory controlled by the Assad regime, the Syrian Observatory for Human Rights (SOHR) reported on Monday.

According to reports, dozens of locals were shot or stabbed to death during the three-week-long massacre, with their bodies dumped in a pit in al-Qaryatayn, a town in Syria’s central Homs province. Government forces have since retaken the area.

The majority of people were reportedly killed on suspicion of “communication and espionage in favor of the regime forces.”

“After the regime retook it (on Saturday), the town’s residents found the bodies on the streets. They had been shot dead or executed with knives,” SOHR’s Abdel Rahman said. “Most of the ISIS fighters who attacked the town a month ago were sleeper cells. … They are from the town, know the town’s residents and who is for or against the regime.”

“The majority of those killed were executed in the last two days before ISIS lost the town again,” he added.

Reuters confirmed at least 60 of the civilian killings, adding that “dozens” of residents remained missing. “More than 60 were dead, while more than 100 others are missing, either kidnapped or killed,” Homs Governor Talal Barazi told the news organization.

While the attack was unexpected and a relative success for the caliphate, the Islamic State continues to weaken amid losses of territory and a collapse in finances, with analysts now saying the caliphate has reverted from a terror state back into a terrorist organization.

The Syrian Arab Army (SAA)—led by its commander-in-chief, dictator Bashar al-Assad—remains one of the many factions attempting to drive ISIS out of the Middle East, alongside the Free Syrian Army and the U.S.-led coalition.

Last month, Syrian government successfully surmounted an ISIS siege on Deir Ezzor, a town in Eastern Syria, and are close to retaking the region entirely.

Meanwhile, the U.S.-led coalition last week celebrated the defeat of ISIS in the Syrian city of Raqqa, while forces in the Philippines also expelled the terror group from their stronghold of Marawi. Islamic State “caliph” Abu Bakr al-Baghdadi had declared Raqqa the capital of the terrorist state in 2014.

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