The "Islamic State of Iraq and Syria" (ISIS) group's chief has urged followers to free detained extremists and family members held at camps in Iraq and Syria in an audio recording released Monday (September 16th).

The recording, purportedly by ISIS chief Abu Bakr al-Baghdadi, was distributed via the Telegram messaging app.

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

In Syria, where ISIS in March lost the last scrap of the territory it once controlled, hundreds of suspected extremists are in the custody of the Syrian Democratic Forces (SDF) which spearheaded the fight against ISIS.

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 north-eastern Syria.

In Iraq, where authorities declared victory over ISIS in December 2017, courts investigating ISIS-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 ISIS links.