Isis, steadily losing against US-backed Iraqi and Syrian forces, is forcibly enlisting young men in territory still under its control to fight for the so-called caliphate.

A statement distributed in villages and towns around Deir Ezzor province on Thursday, shared by local activists, called on able-bodied men aged between 20 and 30 to take up arms against approaching Syrian government forces.

They are expected to report for duty at mobilisation offices within a week’s time, the message said. The majority are expected to be rounded up after Friday prayers – which are compulsory under Isis’s brutal and prescriptive laws.

Liberated from Isis, women burn their burqas and men shave off their beards

Those who do not volunteer themselves will be questioned and punished.

Deir Ezzor city has been besieged by Isis since 2015, and the militants still control large parts of the surrounding province.

In recent weeks, the Syrian army and allied militias have been closing in on the city, which Isis has been unable to defend properly thanks to fighting on several other fronts.

As well as the US-backed Kurdish-Arab offensive on Raqqa and Syrian regime gains in Homs province, the group’s capabilities have been greatly impeded by relentless air strikes.

In pictures: Isis' weapons factories Show all 11 1 /11 In pictures: Isis' weapons factories In pictures: Isis' weapons factories A mortar round fin manufactured by Isis in Gogjali, Mosul, November 2016 Conflict Armament Research In pictures: Isis' weapons factories Isis rocket components discovered in Gogjali, Mosul, Iraq in November 2016 Conflict Armament Research In pictures: Isis' weapons factories Isis mortars discovered near Karamlais, Iraq, in November 2016 CAR In pictures: Isis' weapons factories An Isis rocket launch frame in Qaraqosh, November 2016 Conflict Armament Research In pictures: Isis' weapons factories A memo from Isis' COSQC on quality control at a manufacturing facility in Gogjali, Mosul, November 2016 Conflict Armament Research In pictures: Isis' weapons factories Electrically-operated initiators manufactured by Isis in forces Gogjali, Mosul, November 2016 Conflict Armament Research In pictures: Isis' weapons factories Isis mortar tubes at a manufacturing facility in Karamlais, November 2016 Conflict Armament Research In pictures: Isis' weapons factories An Isis mortar production facility discovered in Gogjali, Mosul, in November 2016 Conflict Armament Research In pictures: Isis' weapons factories An Isis weapons manufacturing facilities near Mosul in November 2016 Conflict Armament Research In pictures: Isis' weapons factories Stocks of French-manufactured Sorbitol, Latvian potassium nitrate and Lebanese sugar at an Isis weapons factory in Iraq Conflict Armament Research In pictures: Isis' weapons factories A destroyed Isis weapons facility in Qaraqosh, Iraq, November 2016 Conflict Armament Research

Forcing grown civilians rather than child recruits to fight is a new strategy for Isis – which observers say shows how vulnerable the group is now.

There are no reliable population statistics available for the area – which has seen intense bombing from the US coalition, Russia and the Syrian government, as well as the mass displacement of people fleeing fighting. But before Syria’s civil war broke out in 2011, Deir Ezzor province was home to 1.4 million people.

Isis lost control of its de facto Iraqi capital of Mosul last month, after sweeping across the border from neighbouring Syria in 2014.

The bloody nine-month offensive claimed thousands of civilian lives, and has left major parts of the city destroyed.

In Syria, Isis is struggling to retain its last urban stronghold of Raqqa; US-backed Kurdish-Arab coalition forces have already succeeded in seizing half of the city since Operation Euphrates Wrath began in June.