Mr David Trimble, on the eve of his party's annual conference, has dismissed the republican movement's professed commitment to peace as little more than a tactic designed to squeeze concessions from the British and Irish governments.
Sinn Fein in turn accused Mr Trimble of dealing a "savage blow" to those who wish to see the Mitchell review break the deadlock over decommissioning and the formation of an executive.
The Ulster Unionist Party leader gave a cool response to a suggestion from his Assembly colleague, Dr Esmond Birnie, that could see Sinn Fein taking its two positions in the executive ahead of IRA decommissioning.
On Thursday night, Dr Birnie floated the possibility of Sinn Fein in government based on a post-dated guarantee that if the IRA did not subsequently begin disarming the Sinn Fein ministers would resign.
Mr Trimble, speaking at Stormont yesterday, described Dr Birnie's suggestion as hypothetical. "It is not my idea. It is not particularly well-judged at the moment."
Mr Trimble, ahead of today's annual conference of the UUP in Enniskillen, Co Fermanagh, released a 26-page document, "Implementing the Agreement", at Stormont. The document, which is being presented as a party position paper to Senator George Mitchell's review of the Belfast Agreement, repeats the UUP view that without IRA disarmament Sinn Fein cannot sit in an executive.
The document stresses that the UUP does not accept the bona fides of the republican movement in its commitment to peace.
"No one should be mistaken - if Sinn Fein were genuine about paramilitary disarmament they would have achieved it by now, just as they could have achieved it in 1995 or 1996," the paper states.
"Their entire commitment to peace and democracy can now be seen as little more than a tactic, a means of squeezing as much as possible out of the governments while reserving the right to go back to the `armed struggle'," the document says.
Mr Trimble, at a press conference to launch the paper, said the agreement "can't work unless it all works", and that included decommissioning. The talks so far between Sinn Fein and the UUP had been mere "shadow boxing". He reiterated the unionist dismissal of any republican "seismic shift" in July when the British and Irish governments appeared prepared to accept that in the event of Sinn Fein entering the executive, IRA disarmament would follow.
"The July paper failed, and if that is the thinking of today I am afraid it is going to fail again. We are not going to buy the same rather dubious animal twice," said Mr Trimble.
He said there would be no softening of the UUP position on guns after the party conference. While insisting on an IRA commitment to disarm, he indicated there could still be movement around the notion of sequencing.
Asked what republicans had to do to persuade him they were genuine, Mr Trimble replied: "Simply that they are going to carry out their obligations, and that they are going to do this on this date, to tell us when they are going to give notice of intention to decommission to General de Chastelain, when they are to supply him with information as to quantities, when they are going to agree modalities with him, when they are going to agree a timetable, when they are going to start implementing that timetable.
" When we have that set out in front of us, then we will have something to talk about."
Senior Sinn Fein negotiator Mr Gerry Kelly said Mr Trimble had thrown down the gauntlet to the two governments and had effectively joined the DUP "in its rejection of the Mitchell review of the agreement". "The UUP is locked in a past which spurns true democracy and believes nationalists and republicans should remain in an inferior place," added Mr Kelly.
The SDLP in a statement said it was vital that pro-agreement unionists "emerge strengthened and encouraged from their proceedings this weekend".
