Human Rights Watch on Friday called on the Kurdish-led forces in northern Syria to ensure the humane treatment of evacuees and recaptured detainees who escaped from a prison complex housing suspected members of the "Islamic State" (IS).

The rights watchdog expressed concern over the fate of the 700 boys who had been living in the prison.

IS militants attacked the complex in the northern city of al-Hasaka on January 20. In the ensuing battle to retake the prison carried out by the Syrian Democratic Forces (SDF) with the support of the US-led coalition, more than 500 people were killed.

"The Syrian Democratic Forces began evacuating men and boys from the besieged prison days ago, yet the world still has no idea how many are alive or dead," said Letta Tayler, HRW's associate crisis and conflict director.

"The detaining authorities in northeast Syria should end their silence on the fate of these detainees, including hundreds of children who were victims of ISIS," she added.

HRW seeks more information following prison siege

The al-Sina'a prison held around 4,000 inmates before the siege, consisting mainly of adult men suspected of being IS members and their families.

HRW said the SDF should allow aid groups access to the remaining prisoners and publicize data on the number of people killed, including children.

Various inmates have been transferred to other sites, including one funded by the UK. "Everyone is in safe places. They receive good care," SDF media commander Siyamend Ali told HRW.

The SDF also said IS had used children as human shields, but added that the Kurdish-led forces had made efforts to protect the children when they recaptured the facility.

HRW questioned both claims, saying the SDF had not given details on what treatment inmates were receiving or the steps they took to protect children during the siege of the prison.

Foreign detainees stuck in northern Syria

The HRW report on Friday also said almost 45,000 men, women and children from nearly 60 different countries were being held in prisons across northern Syria because of suspected connections to IS — the extremist group that ruled large swaths of Syria and Iraq until it was beaten back by the Kurdish-led forces and US-led coalition.

HRW said the "vast majority" of foreign detainees were young children and their mothers.

Part of the problem is the lack of willingness from most countries to take back their citizens who are being held in prisons.

"This assault should be a wake-up call to countries that outsourcing responsibility for their nationals won't make this problem go away," HRW's Tayler said.

"It will only increase the suffering of these detainees, most of them young children, and deprive ISIS victims of justice," she added.

Edited by: Sean Sinico