(Recasts with attempted storming of prison)
BEIRUT, May 6 (Reuters) - Syrian government forces fired tear gas and live ammunition into Hama prison on Friday as they tried to put down a revolt by its inmates, a monitoring group said, and an opposition source said some prisoners had choked from the fumes.
The prison has been surrounded by government forces since hundreds of prisoners rioted and seized several guards on Monday, the Syrian Observatory for Human Rights said. Friday's fighting was the second attempt to storm the prison in the western part of the country, the Britain-based group reported.
A Syrian opposition official in contact with prisoners said security forces had fired an unidentified gas into the facility, causing 25 cases of suffocation but no deaths.
The official said Syria's main opposition body, the High Negotiations Committee, had informed the United States and the U.N.'s Syria envoy about the situation. The HNC earlier said it held the international community responsible for a "massacre" of inmates it expected government forces to carry out.
The Syrian interior ministry has denied "reports ... about Hama central prison", but has not elaborated on the issue since Monday.
Syrian insurgent groups, including the powerful Islamist Ahrar al-Sham, threatened to shell all the bases of government forces in Hama province if Damascus did not meet the demands of the inmates.
The prisoners rioted to protest the attempted transfer of some of them to Sednaya military prison, north of Damascus, and because trials had been postponed, the Observatory said . The HNC said they wanted better conditions in jail.
Earlier on Friday, the Observatory said, negotiations had led to the release of dozens of prisoners.
Those held in the jail include political and Islamist prisoners. International rights groups say thousands of detainees are held in Syrian government prisons without charge and many of them are subjected to torture, a claim denied by the authorities.
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. (Reporting by John Davison and Tom Perry; Editing by Larry King)
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