Northern Ireland's political future must be based on the Good Friday Agreement, the secretary of state has said.

John Reid was speaking after holding talks in Dublin with Irish Foreign Minister Brian Cowen on Thursday as the government considered how to respond to the threat issued by Ulster Unionist leader David Trimble.

Mr Trimble announced on Saturday that he had set an 18 January deadline for republicans to demonstrate they had left violence behind.

If there is no move by republicans, UUP ministers will resign from the Northern Ireland Executive, collapsing the power-sharing government.

John Reid said no alternative to Agreement was on offer

Speaking after Wednesday's meeting, Dr Reid said the parties must work to the Agreement.

"If there is an alternative to the Good Friday Agreement, I have never seen it, no one has ever explained it to me," he said.

"We are all open-minded people, but as far as I am concerned the future of Northern Ireland is based on the Agreement that has been reached.

"It has been backed by the people of Ireland, north and south of the border, by all the major political parties who have sustained it up to this point and, quite apart from that, there can be no sustainable future for Northern Ireland unless it's based on the principles that are enshrined inside the Agreement."

Dr Reid added he still believed both unionists and republicans were committed to the process, although both needed reassurances.

Mr Cowen also emphasised the Irish government's determination for the full implementation of the Agreement.

Move 'would hurt process'

Meanwhile, speaking on Wednesday the US Government's advisor on Northern Ireland, Richard Haass, said any move to withdraw from the executive would hurt the peace process.

Richard Haass: Concerned about developments

Speaking to reporters in Washington he said: "We view paramilitaries and the sectarianism that allows them to thrive as the chief obstacle to normalisation.

"That said, I question how setting deadlines for withdrawing from the institutions helps us address this problem.

"In fact I am concerned that this new deadline will increase the sense of crisis, increase polarisation, undermine trust and make it all the more difficult to focus on what should be everyone's main task - namely, making the institutions of the Good Friday Agreement work."

Sinn Fein to meet Ahern

On Thursday a Sinn Fein delegation is to meet the Irish prime minister, Bertie Ahern, in Dublin.

Party president Gerry Adams, Northern Ireland education minister Martin McGuinness and health minister Bairbre de Brun are to attend the meeting.

Sinn Fein has condemned the Ulster Unionist threat to leave the executive and another move to immediately impose a sanction of boycotting North-South Ministerial Council meetings in Sinn Fein is involved.

Options

The governments are closely examining their options after any unionist withdrawal takes place.

Following the Ulster Unionist ruling council's decision on Saturday, the alternatives include suspending the institutions or calling an early election.

The governments could take the view that a spring poll will help resolve the current instability.

However, they may also fear that it will damage the political centre ground so badly that it would make any future deal between the parties impossible to reach.