Isis has killed at least 128 people in a Syrian town deep inside regime-held territory, a human rights monitor has said.

Government forces managed to retake Al-Qaryatayn over the weekend, the Syrian Observatory for Human Rights (SOHR) confirmed on Monday.

Rumours of execution-style killings of captured soldiers or civilians suspected of links to the government had not previously been verified.

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Al-Qaryatayn, some 300 kilometres (190 miles) from one of Isis’s strongholds of Deir Ezzor, was recaptured by militants in an unexpected attack far from the front-line fighting earlier this month.

An Associated Press video, filmed as Syrian government troops recaptured Al-Qaryatayn, showed several bodies in the streets of the town. In the video, a town resident says Isis “monsters” killed more than a 100 people, including soldiers and civilians.

“These are people who don’t know God, they don’t know anything. They killed children and women with knives, they beat women, broke their arms,” he said.

Talal Barazi, the governor of the surrounding Homs province, said most of the bodies were of townspeople who were government employees or were affiliated with Syria’s ruling Baath party.

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He said the killings went on for the three weeks that Isis held the town and “terrorised” its residents. “It is a shocking massacre,” he said.

At least 153 Syrian army soldiers and fighters from allied militias, such as Lebanon’s Hezbollah, lost their lives over five days of heavy clashes for the Homs province town, the UK-based SOHR previously said.

It is still not clear how militants were able to carry out a successful offensive so far away from supply lines. Isis has been known to use sleeper cells, often dressed in the uniforms of government or Syrian Democratic Forces (SDF) soldiers, to carry out attacks behind enemy lines in the past.

While the militants’ counter offensive took Bashar al-Assad’s forces by surprise, it came as the group continued to steadily lose control of its once huge empire across both Syria and Iraq.

Last week, Isis’s de facto capital of Raqqa was declared free of the jihadis after a four-month-long battle by US-backed Kurdish-Arab SDF.

The Syrian government managed to break through the Isis siege on the eastern town of Deir Ezzor in August. Despite a ferocious stand by the militants, they will be unable to hold out against regime ground troops and Russian air power for long.

Isis was previously forced out of Al-Qaryatayn in April 2016 after seizing it in August 2015.