Northern Ireland Secretary John Reid is to invite the political parties to meetings aimed at defusing tension over the Ulster Unionists' decision to set a deadline for peace.

Party leader David Trimble announced on Saturday that UUP ministers would resign from the Northern Ireland Executive, collapsing the power-sharing government, if republicans had not proved they were committed to peace by 18 January.

The decision was taken at a meeting of his party's ruling council at which hardliners pressed Mr Trimble to adopt a tougher line against Sinn Fein following claims of republican paramilitary activity.

Mr Trimble said his party would also impose the immediate sanction on republicans of ending Ulster Unionist participation in North-South Ministerial Council meetings in which Sinn Fein was involved.

It could be that the people in the IRA move unequivocally into Sinn Fein and devote themselves purely to democratic means

David Trimble

He added the Ulster Unionists hoped to get into talks with the British Government and the other political parties over the next three months to "see if there is a viable basis for future government in Northern Ireland".

However, Sinn Fein's Martin McGuinness accused the Ulster Unionist party of assaulting the Good Friday Agreement.

The Education Minister said many people viewed Jeffrey Donaldson as the real Ulster Unionist leader but warned Mr Donaldson could not ignore Sinn Fein.

Meanwhile, the Democratic Unionist Party leader Ian Paisley has called for the assembly to be dissolved so elections can be held immediately.

Speaking on Sunday, Dr Reid said he could understand the Ulster Unionists' need for reassurance but he called on all those involved in the political process to consider what was at stake.

He told the BBC's On the Record programme there was no reason why IRA decommissioning and the disbanding of the IRA could not be completed by 18 January.

"It could be that the people in the IRA move unequivocally into Sinn Fein and devote themselves purely to democratic means," he said.

Martin McGuinness: Accused the UUP of 'assaulting' the Agreement

Mr Trimble said Sinn Fein should have been preparing the IRA for the "inevitable".

He said: "This process cannot be sustained as things stand at the moment."

Deputy leader of the nationalist SDLP Brid Rodgers condemned the Ulster Unionist threat as "wrong".

"Does he not recognise the difference that the past five years have made to the people of Ireland, north and south," she said.

"It is clear now that Mr Trimble has joined the "no-men" who set out from the very beginning to wreck the Agreement.

"They didn't support power-sharing and they certainly did not support the principle of equality."

The Irish Government, reacted with dismay to Mr Trimble's announcement of the deadline.

Irish Prime Minister Bertie Ahern said the decision of the Ulster Unionist Council was "regrettable and a cause for concern".