Government forces have encircled a prison in the central Syrian city of Hama as prisoners reportedly took captive the warden and several guards during heavy clashes.
The prison uprising broke out in response to the government's plans to forcibly transfer several detainees to Sednaya, an infamous prison near Damascus, sources inside the jail told Al Jazeera.
Government forces have surrounded the prison and fired tear gas in an effort to quell the unrest, according to sources in the jail and the UK-based Syrian Observatory for Human Rights.
|Thousands of photos 'prove crimes against humanity' in Syria
The Observatory said the prisoners, many of whom are detained without charge, demanded "basic rights", including a fair trial or release.
In response, the Ajnad al-Sham rebel group vowed to attack government forces and armed groups loyal to Syrian President Bashar al-Assad if the prisoners' demands were not met.
In a statement released on Monday night, Ajnad al-Sham claimed the government had threatened to "storm the prison and execute all the prisoners without a trial".
The group added: "Our brothers inside the prison are still carrying out the uprising and have taken complete control of the building. We in Ajnad al-Sham declare... our full readiness to strike [government-allied] militias in Maharda and al-Suqaylabiyah."
READ MORE: Speaking out on 'sadistic' Syrian jails
The state news agency SANA said an official in the interior ministry had dismissed claims "in some media" about an uprising in the prison.
The Syrian conflict started as a largely unarmed uprising in March 2011, but it quickly morphed into a full-on civil war between government forces and rebel groups.
United Nations special envoy to Syria Steffan de Mistura recently estimated that more than 400,000 people had been killed throughout the fighting.
International and Syrian human rights organisations have decried conditions inside Syrian prisons before and during the war.
Between March 2011 and the end of 2015, the Syrian Network for Human Rights documented the arrest and detention of more than 117,000 people.
READ MORE: Up to 7,000 Syrians tortured or killed in detention
In a December 2015 report, Human Rights Watch (HRW) said its researchers had found "evidence of widespread torture, starvation, beatings, and disease" in government jails and detention centres.
"Many of the former detainees who were held in these nightmarish conditions told us they often wished they would die, rather than continue suffering," said Nadim Houry, deputy Middle East director at HRW.
"They begged countries involved in seeking a peace process to do everything they can to help the people still being held in Syria."
Source: Al Jazeera