Syria's Kurds are struggling to hold 12,000 Islamic State (IS) suspects in prisons that are under threat from IS attack and takeover by Syrian Government forces.

Key points: Some Islamic State inmates have already escaped

Some Islamic State inmates have already escaped One doctor says prisoners are dying at a rate of about one each day

One doctor says prisoners are dying at a rate of about one each day There are at least seven Australian IS suspects in Kurdish prisons

Turkey launched an offensive against Syrian Kurdish areas two weeks ago after President Donald Trump said US troops based there would leave and would not interfere with a Turkish attack, prompting fears that Kurdish fighters guarding IS prisoners would be diverted to the frontlines of a Turkish invasion.

The Kurds have since cut a deal with the Syrian Government, allowing Government troops to move in to defend the area.

IS inmates have already escaped, and, in one jail complex, injured children appear to be suffering from malnutrition.

In the hospital ward of a Kurdish jail, prisoners in orange suits carry an emaciated boy to an empty spot on the floor. His legs have withered to little more than bone, pinned together with metal rods.

The guards aren't sure how much the prisoners know about Turkey's invasion. ( ABC News: Tom Hancock )

Feras Mohammed is the son of an IS fighter. He says he's 14 years old, but he looks younger.

He's being cared for by a former IS doctor, Khomedi Shukri al Mohammed. He said patients were dying at a rate of about one each day and Feras would be next without more treatment.

Fighting has damaged the city of Hassaka's water supply and drinking water is now being trucked into the prison from a nearby dam. The water is dirty and that's making Feras even sicker.

The IS prisoners held by Syria's Kurds have been here for at least eight months, since the fall of the last piece of the group's caliphate at the town of Baghouz.

Turkey's invasion of north-east Syria now threatens the Kurds' ability to hold them.

Some Islamic State inmates have already escaped. ( ABC News: Tom Hancock )

Eighty per cent of the guards here have gone to the frontline, said prison head Sarhad Amuda.

"They are now always trying to break down the doors," he told the ABC.

"Just a few days ago one of them pretended to be sick, and when our comrades tried to bring him to the hospital they were attacked and they tried to escape. In recent days this is always happening.

"This prison's been operating for six months and we haven't seen anything like this. In the last 10 to 15 days they are always hyperactive, always awake.

"If you open the door everybody is coming to the door. Before this it wasn't the case."

There are about 12,000 Islamic State suspects in prisons in the Kurdish region of Syria. ( ABC News: Tom Hancock )

The guards aren't sure how much the prisoners know about Turkey's invasion, or the arrival of soldiers loyal to Syrian President Bashar al-Assad.

Before the ABC was allowed in, journalists were required to agree not to tell prisoners about current events in case it sparks a mass breakout.

An IS sleeper cell has already attacked one prison in Hassaka, detonating a car bomb outside the jail. The prison walls weren't breached.

"They don't know exactly what is going on outside the prison — they know something is going on, that some cells are trying to take them out from prison. They feel something is happening outside," Mr Amuda said adding that if they knew the situation it would be very hard to control them.

Egyptian prisoner Yasser Mohammed Abdul Azim said he desperately wanted to face trial so he could see his family again.

"Why we are still here in prison? Why we are not out? Why we are not in court?" he said.

"We wish to be with our families. We wish to have another chance to show the world we are not guilty."

There are at least seven Australian IS suspects in Kurdish prisons. Several have been shifted to more secure prisons in recent days, as the war comes closer.