Sinn FEIN is seeking a meeting with the Ulster Unionist leader, Mr David Trimble, to discuss his decision to write a post-dated letter of resignation from the new Northern Ireland executive, to take effect if the Provisional IRA does not begin decommissioning by early next year.
Sinn Fein believes that this is outside the terms of the Belfast Agreement and the Mitchell review.
The Sinn Fein leader, Mr Gerry Adams, welcomed the UUC's decision to enter government with his party, but said: "I am disappointed that Mr Trimble has stepped outside the Mitchell review and the Good Friday agreement by unilaterally introducing a new element, a new deadline, which seeks to dictate and totally undermines and contradicts the agreed role of the Independent International Commission on Decommissioning."
Mr Adams said the UUP leader's approach would "fuel uncertainty and keep alive the hopes of rejectionists inside and outside the UUP".
He added: "Mr Trimble is the best judge of how to manage his own party and I know he has difficulties. But unilaterally introducing new elements and setting up new hurdles is not the way to do business."
Sinn Fein's chief negotiator, Mr Martin McGuinness, said yesterday that decommissioning had to be a voluntary exercise. It was incumbent on everybody to move forward in a positive and constructive way, but the demand by unionists had not been helpful to the process. It would make decommissioning more difficult.
He said the step taken by the unionists on Saturday was not "in my opinion, a decisive step forward. It was a jump forward to uncertainty. We need to remove that uncertainty. We need to know there is going to be devolution, that institutions are going to be created and continue, and that the politicians are going to work in honest endeavour to create the conditions to make it possible for the armed groups to decommission. That is what Sinn Fein is committed to."
He added that the jury was out on what the unionists were at. "I give them credit for the decision they took yesterday, but people should not, under any circumstances, enter new elements into this that have not been agreed between us during the course of our discussions in recent days."
Mr Adams described the UUC vote as progressive but added: "It is short-termism and fragmented, instead of strategic and holistic."
Sinn Fein Assembly member for West Tyrone, Mr Pat Doherty, said the area's UUP MP, Mr Willie Thompson, who is to resign his party membership this week, should also resign from Westminster.
"The vast majority of people in West Tyrone at every election have voted for pro-agreement parties. If Willie Thompson really wants to see if the people of West Tyrone want Sinn Fein in the executive, then he should resign and let the people decide.
"I can guarantee him that not only will we see Sinn Fein in the executive, we will have a Sinn Fein MP in West Tyrone."