Mr David Trimble has said his party will quit any new Executive if the Provisional IRA fails to honour its pledges on disarmament.
He insisted that the Ulster Unionists would not be locked into an Executive with Sinn Fein if the IRA does not put arms beyond use in a credible and verifiable way.
The UUP leader said he was considering putting to the Ulster Unionist Council next Saturday a proposal to return to government with Sinn Fein while reserving the right to review its participation depending on progress on decommissioning.
Mr Trimble acknowledged there was concern within his party that "if somehow we move on this we are then locked into a situation".
He said: "That is simply not the case. We retain completely freedom of action ourselves. From our point of view if there is any delay, if there is any footdragging, then that is going to have serious political consequences."
Speaking on BBC Radio Ulster's Inside Politics, he said that while the Provisional IRA's statement on arms broke new ground, one of its "weaknesses" was the failure to indicate any timetable for putting the weapons beyond use.
Attention on decommissioning should also focus on loyalist paramilitaries, he added. In an appeal to rank-and-file UUP members, he said his critics could be only negative.
"The amazing thing in this situation is that no alternative is being presented. There is a road, a road that we are following, that leads to peace, that leads to democracy."
Writing in Saturday's Newsletter, Mr Trimble said the Provisional IRA's process of putting its arms beyond use was actual decommissioning.
The suspension of the devolved Executive in February had forced the IRA to make "an unprecedented offer", he said. "There is no difference in practical terms if IRA weapons are made permanently inaccessible or unusable - this is decommissioning. The weapons are destroyed."
Mr Trimble said UUP negotiators had made "real progress" in the campaign to retain the RUC's name. However, he warned if unionists rejected the Belfast Agreement, it would be replaced by "green-tinged rule" and they would be "in the wilderness for years".
He predicted that Sinn Fein could eventually become the largest nationalist party in the North and a substantially worse deal would await unionists.
DUP secretary Mr Nigel Dodds accused Mr Trimble of "scaremongering". The UUP leader offered only "threats and blackmail" and his political arguments were "threadbare".
Mr Dodds said: "The reality is David Trimble came out of the Executive in February because no IRA arms were produced, there was no IRA decommissioning.
"Now he expects his party to endorse a proposal to go back into government three months later with IRA/Sinn Fein with no arms up front and on what basis? On comments made in the House of Commons by Tony Blair that he believes decommissioning will take place."
The Rev Ian Paisley criticised the UUP leadership that if unionists did not accept this deal, the alternative was even worse. "This is laughable. They say Dublin will have too much influence. Dublin has joint authority at the moment."