"The prisons, the prisons, soldiers of the caliphate!" said Abu Bakr al-Baghdadi, the elusive head of the group which once controlled swathes of Iraq and Syria, in a recording purportedly by the IS chief distributed via the Telegram messaging app.

"Do your utmost to rescue your brothers and sisters and break down the walls that imprison them," he said in the recording released by the jihadists' propaganda organ.

Thousands of fighters from the jihadist group are held in overcrowded prisons in Iraq and Kurdish-held regions of Syria, while tens of thousands of their family members are held in camps.

In Syria, where IS in March lost the last scrap of its self-declared "caliphate", hundreds of suspected jihadists are in the custody of the Kurdish-led Syrian Democratic Forces which spearheaded the fight against IS.

Kurdish authorities say 12,000 foreigners from as many as 40 countries, including 4,000 women and twice as many children, are in camps it runs in northeastern Syria.

The Red Cross said in early July that conditions in the camps were "apocalyptic", urging countries to quickly repatriate family members of suspected foreign fighters.

In Iraq, where authorities declared victory over the jihadists in December 2017, courts investigating IS-linked crimes have condemned more than 500 foreigners to prison and death sentences.

Iraqi authorities do not release figures on detainees in their prisons, but observers say some 20,000 people are in their custody over suspected IS links.

In April, IS released a video urging its supporters to continue their battle and showing Baghdadi for the first time in five years.

Despite their territorial defeat, the jihadists have continued to operate sleeper cells and launch attacks in both Iraq and Syria.