“He was saying: ‘There’s a lot of people dead in front of me … I’m scared I might die anytime … I don’t know what to do. Please help me,’” she said.
Ms. Tayler said the boy, a foreigner, reported that the prison was “getting hit from every side.”
Last May, a United Nations human rights report said the conditions under which children were held in northeast Syria meet the threshold for torture, and inhuman and degrading treatment under international law. It described overcrowded conditions, no access to sunlight, malnourishment and untreated injuries.
Ms. Tayler said the crisis happening now could have been averted if the children’s home countries had agreed to repatriate them.
“Detention should be an exceptional measure of last resort,” she said on Twitter. “Instead, foreign countries dumped responsibility for these children on the NE Syrian authorities. If anything happens to these boys during this prison assault, the boys’ home countries will have children’s blood on their hands.”
A propaganda video released by the Islamic State on Sunday showed more than a dozen men identified by the S.D.F. as kitchen workers being held captive by masked ISIS gunmen.
In Iraq, which is also holding its own ISIS detainees and is still battling a persistent jihadist presence, the government has stepped up security at its prisons.
The coalition said it was confident that the attack in Hasaka would not pose a significant threat to Iraq or the region but was still assessing whether the Islamic State was planning any further attacks on detention facilities in Iraq and Syria.