AMMAN (Reuters) - Conditions were worsening in a Syrian prison where nearly 800 mostly political detainees have launched a revolt, with security forces failing in an attempt to storm the facility overnight, rights groups and activists said on Saturday.

The prison has been surrounded by government forces since Monday after inmates rose up and seized several guards in protest at the transfer of five detainees sentenced to death to a feared high security prison, according to rights activists in touch with inmates and with New York-based Human Rights Watch.

Security forces on Friday fired dozens of tear gas bombs and used rubber bullets in an attempt to end the rebellion.

Videos released by inmates on social media showed several men gasping for breath and coughing and saying they were suffocating from “poisonous gas”, with gunshots heard in the background and fires breaking out in the corridors after the gas cannisters were launched.

Syrian rights activist Mazen Darwish, a former detainee in the prison and in touch with prisoners said conditions were fast deteriorating with electricity and water cut for three days amid food shortages and serious medical conditions among some of the inmates.

“Inmates are running out of food and water and even medicines are no longer being given to those in serious conditions,” Darwish said, adding prisoners wanted the Syrian Red Crescent to mediate on their behalf after an earlier deal that released at least 46 detainees before talks broke down.

Inmates have demanded the release of political detainees held without charges and say their fears mainly stem from a wave of executions that could follow if they were to be transferred to the Sadnaya military prison, north of Damascus.

International rights groups say thousands of detainees are held in Syrian government prisons without charge and many of them are tortured to death, a claim denied by the authorities

Human Rights Watch expressed concern about the safety of the hostages and said an attempt to retake the facility risked high casualties.

“This standoff should not end in a bloodbath,” Nadim Houry, deputy Middle East director at Human Rights Watch was quoted as saying in a statement late on Friday.

“The situation in Syria’s detention facilities and prisons is deeply unstable and prison conditions should be a priority for the international community.”

Syria’s military field courts have secret, closed-door proceedings that do not meet basic fair trial standards, HRW said in the statement.The Syrian interior ministry has denied “reports ... about Hama central prison”, but has not elaborated on the issue since Monday.

Earlier this year, United Nations investigators said detainees held by the Syrian government were being killed on a massive scale amounting to a state policy of extermination of the civilian population. [L8N15N2Y2]

The Syrian conflict began in 2011 with popular protests against President Bashar al-Assad and spiralled into civil war after a crackdown by security forces.