The Ulster Unionists have withdrawn from any political talks which do not primarily deal with the existence of the IRA.
Leader Mike Nesbitt said they would not discuss the Stormont House Agreement which tackled outstanding peace process issues until the paramilitarism had been resolved.
Intensive talks were launched by the British and Irish governments after revelations the IRA still existed and members shot a man dead in Belfast last month.
Mr Nesbitt said: "This is the issue and the only issue that we will speak on.
"We will consider going into the session which deals with the IRA. We will consider it but we need to see the papers and the terms of engagement.
"That is what we said about going into these talks. Who would be there, what are the terms of engagement and what is the agenda?
"The fact that no other party supported us and said 'let's change this proposed agenda, forget the Stormont House Agreement until we resolve the issue of Sinn Féin being in denial about the IRA' is frankly appalling."
The ministerial Executive at Stormont has been under threat of collapse since police said IRA members were involved in the murder of Kevin McGuigan in east Belfast last month.
Police believe Mr McGuigan was killed by individual members of the Provisional IRA in revenge for the death of prominent republican Gerard "Jock" Davison in May.
Although PSNI Chief Constable George Hamilton has insisted the IRA is not back on a war footing, disclosure that the organisation still exists has rocked the political establishment.
First Minister Peter Robinson said: "If the Assembly and Executive falls we are probably talking about the best part of a decade before it would ever be revived.
"These are extremely important talks. We enter it in a very serious manner."
