Ulster Unionist leader Mr David Trimble is giving the British and Irish governments a deadline of the end of the month to produce proposals on how they would deal with paramilitary violence.
Emerging from a hour-long meeting with the Taoiseach Mr Ahern and British Prime Minister Mr Tony Blair at Hillsborough Castle, the Northern Ireland First Minister said both governments needed to spell out how they would have to draw a line on paramilitary behaviour.
"They have to recreate this confidence and do it quick," he said.
"If we don't see significant progress on this by (the British) government in the course of the end of this month before parliament goes into recess, then in the absence of an actual clear move by the government there is going to be a very serious problem indeed."
Mr Trimble refused to spell out clearly what he would do in the absence of any movement.
However, before leaving to brief party colleagues at a nearby hotel on his meeting, he said that both governments needed to produce a clear definition of what this required of paramilitaries and what action they would take if ceasefires were broken.
Mr Trimble was commenting after the Taoiseach and Mr Blair held a series of meetings with Northern Ireland pro-agreement parties in a bid to restore confidence in the peace process.
SDLP leader Mr Durkan speaking after his meeting with the two governments criticised Mr Trimble's threat to pull the process down. He said: “People should not be walking away from these talks nor should they be threatening to walk away from the institutions.”
Pro-agreement parties and both Governments have begun talks earlier in an effort to head off a deepening crisis in the North's peace process amid warnings that increased paramilitary activity was going unchecked.
A small but vocal loyalist protest heckled and jeered political leaders as they entered Hillsborough Castle amid tight security.
Extra police have been drafted in and roadblocks have been mounted in and around the north Co Down village.
Protesters which included hardline Orangeman Mr Mark Harbinson called Mr Trimble a traitor.
Arriving for the talks, SDLP leader Mr Mark Durkan warned of the increased level of paramilitary activity that was going unchecked.
He said it was often the situation that the paramilitaries were be praised for not doing worse.
Mr Durkan called on both the British and Irish governments be "less in denial" about paramilitary activity.
"Where there is activity in the dark recesses of this process we have to shine a light into those activities and expose them - not put the lights out."
As the Sinn Féin delegation led by Mr Adams arrived, there were chants of "murdering scum".
Mr Adams rejected calls from unionists for his party to be thrown out of government: "Our participation in government is because we have a mandate.
Mr Adams conceded that republican paramilitaries have been involved in recent disturbances.
"I accept that all of the armed groups and that means the British army and the police service not only remain in existence but are hyperactive in terms of their presence and attitude in republican heartlands," he said.
As hardline unionists continue to demand the removal of Sinn Féin from the power-sharing executive, the Taoiseach, Mr Ahern, and British Prime Minister Mr Tony Blair will chair the "mini-summit" against a backdrop of street violence, alleged breaches of the IRA's ceasefire and pressure on the First Minister's position within his own party.
Mr Blair and Mr Ahern are expected to hold a series of individual and joint meetings at Hillsborough Castle with the parties this afternoon and a final round-table session involving all sides this evening. Officials in London and Dublin were not optimistic about the prospects of a communiqué being issued on behalf of all the parties at the end of today's talks.
Mr Blair also insisted yesterday that there was "no level of [paramilitary] violence" in Northern Ireland "which can be tolerated." His comments came as Northern Ireland Deputy First Minister Mark Durkan said the British and Irish Governments needed to be "honest and credible" about violence on both sides.
The leaders of the two main churches in Northern Ireland appealed today to all the political parties in the North to work through the present difficulties in the peace process.
Catholic Primate Archbishop Sean Brady and Church of Ireland Primate Archbishop Robin Eames issued their joint message as they were presented with honorary degrees by the University of Ulster.
PA
