A group of loyalist prisoners in Maghaberry jail, Co Antrim, tried to kill a "Real IRA" inmate, according to republican campaigners.
The Irish Republican Prisoners' Welfare Association last night claimed a gun was put to the head of John Connolly but the weapon jammed.
Connolly (27) from Newtownbutler, Co Fermanagh, who is serving 14 years for possession of a barrack-buster mortar, was then allegedly beaten by seven loyalists. Another prisoner, who came to his rescue, was also assaulted. Both republican and loyalist prisoners in Maghaberry are demanding segregation.
A spokesman for the Northern Ireland Prison Service confirmed two prisoners were assaulted and that the Police Service of Northern Ireland was investigating the incident.
The prisoners sustained bruising and one was treated for a cut to his head, he said. The inmates who were suspected of carrying out the assault were removed from the scene and taken to the jail's special supervision unit, he added.
He said there was "no evidence" to support the allegation that a gun had been produced in the attack.
Ms Marion Price of the Irish Republican Prisoners' Welfare Association said the situation in Maghaberry was now "extremely serious".
"John Connolly was returning to his cell after making a phone call to his family when a group of loyalists walked into his cell and put a gun to his head.
"When the weapon jammed, they beat him with tins of food which were in his cell. Another republican prisoner who came to his rescue was also assaulted. Tensions were already high in Maghaberry but are now at breaking-point.
"It's a very serious situation when loyalist prisoners have a gun."
LVF leader Billy Wright was shot dead by INLA inmates while a prisoner in the Maze in 1997.