Ulster Unionist party leader David Trimble has told his party's annual general meeting that republicans must act to restore confidence in the power-sharing executive at Stormont.
He said that devolution will not be reinstated in Northern Ireland unless the IRA goes away.
"We want the Assembly back but it will not happen unless republicans act,'' he said.
"Their act must start with proper open decommissioning and continue with saying the war is over and ensuring that it will not start again - the IRA does have to go away.''
Mr Trimble told about 860 party delegates that the people of Northern Ireland wanted stability and peace.
But he added: "Without republicans changing, acknowledging that they got it so dreadfully wrong, making amends, in deed if not words, the future is bleak. We have tried, tried and tried again to wean republicans from violence.
"If we fail again the world will know that republicans, not unionists have blocked the path to peace, blighted hopes for the future, frustrated the agreement. The onus is on them. They know what has to be done. The time for talk is drawing to an end.''
Mr Trimble was re-elected as leader in an unopposed vote earlier at the meeting.