An Australian teenager taken to Syria as a child is feared dead after languishing in an adult prison for three years.

Yusuf Zahab was 11 years old when he was taken to Syria in 2015 by relatives who had been radicalized. He was imprisoned without charge in 2019.

His Sydney-based family say they are “heartbroken and angry” because the 17-year-old had begged for help for years.

Australia has previously said it is too dangerous to repatriate its citizens.

It is unclear exactly when or how Yusuf is suspected to have died. Rights groups say he was injured in January when the Islamic State group attacked a prison in Syria's Kurdish-controlled north-east in a bid to free its fighters.

Kurdish-led forces, with US support, battled the militants for a week, leaving more than 180 people dead.

At the time, Yusuf told family in voice recordings he was scared he “might die at any time” as fighting intensified.

“I lost a lot of blood... There are no doctors here, there's no one who can help me,” he said.

Yusuf was among around 850 children who had been detained at the prison after the fall of IS in 2019 in case they posed a risk, the UN children's charity Unicef said.

His family described him as a happy and compassionate child who loved sport, school and playing with his cousins.

“Even in the final messages we received from Yusuf, he asked us to tell his mum that he loved and missed her,” they said in a statement.

“Yusuf didn't need to die.”

They said Australia's previous government “knew about Yusuf's predicament for more than three years” before being leaving office in May, adding: “We are unaware of any efforts to support, care or inquire about him.”

The family and advocacy groups Human Rights Watch and Save the Children have pleaded for Australia to repatriate women and children who remain in Syria.

Australia evacuated eight orphans in 2019, but at least 63 Australians remain stuck in Syria. Of them, up to 40 are children, the groups say.

“We have to remember these children were brought there against their will or were born over there,” Human Rights Watch's Sophie McNeill told the ABC.

Australia's Department of Foreign Affairs and Trade said it is working to confirm that Yusuf has been killed, but his family has been offered consular assistance.

The government remains “deeply concerned” about Australians in northeast Syria but its assistance is “extremely limited” due to security dangers, the department said in a statement. (BBC)

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