At least 15 prisoners were killed over the weekend in clashes between rebels and troops at the central prison in Syria's northern town of Aleppo, a watchdog said on Tuesday.
Citing phone calls from sources inside the prison, the Syrian Observatory for Human Rights said "at least 15 prisoners were killed in shelling of the prison, which rebel fighters have besieged."
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It was unclear whether rebel or regime fire was to blame for the deaths on Saturday and Sunday.
Fighting has raged at the facility since earlier this month, with rebels managing to breach the compound before being pushed back.
Some 4,000 prisoners, including Islamists and common criminals, are being held at the facility, which is on the outskirts of Aleppo.