The Democratic Unionist Party will not share power with Sinn Féin until "the durability" of any IRA decision to decommission and end its operations is verified, Mr Peter Robinson has declared.
Speaking in Washington, Mr Robinson said it will take some weeks, or months, for the IRA to decommission its weaponry. "Therefore, clearly, time is required."
The declaration will cause difficulties given the Taoiseach's attempt to get the Northern Ireland Assembly and Executive restored "seamlessly" if the IRA can be brought to decommission its arms.
Speaking at Washington's National Press Club, Mr Robinson said the IRA would have to destroy its weaponry, end all paramilitary activity and all criminal involvement.
He said the head of the International Body on Decommissioning, Gen John De Chastelain, should then provide evidence that decommissioning was in his view complete.
The four-strong Independent Monitoring Commission, PSNI Chief Constable Hugh Orde and the British army commander in Northern Ireland would also have to agree.
"We would also be getting stuff back from our own people on the ground because they would know the activity the IRA is at in their areas," said the MP.
"The summer might be a good period to wait and see how they behave over the so-called marching season. Time is required and it is the clarity, the quality and the durability of what they say and do that will be the key," he said.
He rejected charges that delay would allow the DUP to bank IRA concessions and still refuse to share power with Sinn Féin.
"It is an issue of honesty. If we committed ourselves to do something in those circumstances and we didn't, then I think that the electorate might have something to say about it."
The Belfast meeting, which is seen by Mr Ahern and Mr Tony Blair as an opportunity to fast-track progress, is not "simply hype", said Mr Robinson.
"I am pretty sure that they come with good intention. I think that the intention is to make it abundantly clear that paramilitary violence has to be a thing of the past.
"If they do that job on Tuesday and if they ram that message home, they will have the support of the overwhelming majority of the people of Northern Ireland".
