The Taoiseach has said that a republican move on decommissioning within 10 days is the only way to avert a crisis in the North's political process from which it would be impossible to recover.
Placing responsibility primarily on Sinn Fein to avert political disaster, Mr Ahern said that the inspections of arms dumps which had taken place so far were "not enough".
In a blunt message to the republican movement, he said that the Ulster Unionist Party leader, Mr David Trimble, needed stronger action than in the past, and progress would have to be "very clear".
Acknowledging Mr Trimble's precarious position within the UUP, he said Mr Trimble had been "brave", but his hands were "tied". Emphasising the need for a substantial IRA move, he said: "A process with a vague timetable would not get us anywhere."
The Taoiseach's remarks in the Dail yesterday represent a significant shift from the Government's previous stance of avoiding placing responsibility for progress on any one party. They follow soundings taken by the Government since the election in the North, which saw Sinn Fein strengthened and, in the eyes of senior Government figures, less willing to compromise than before.
Monday's talks in Downing Street heightened Government concern that Sinn Fein's electoral success may have reduced the prospect of republican compromise. Indeed, the Taoiseach remarked that there had been a "rolling back" of past progress since the election.
Mr Ahern will today meet the Northern Secretary, Dr John Reid. Dr Reid and the Minister for Foreign Affairs, Mr Cowen, are expected to begin detailed discussions with the Northern parties in Belfast before the end of this week.
Mr Ahern yesterday painted a doomsday political scenario which he warned would kick in at the beginning of next month if there was no progress. He said that Mr Trimble would resign by his self-imposed deadline of Saturday week and that the Deputy First Minister, Mr Seamus Mallon, would probably also resign.
"Then there would be a six-week period of stalemate during the worst time of the year in the North, following which the Secretary of State would have to make a decision on an autumn election."
An election would produce more polarisation and intransigence, the Taoiseach said. He added: "It is hard to imagine the difficulty of trying to bring it back together again. Quite frankly, I do not think it is possible."
He made it clear that he now saw decommissioning, rather than the other issues of policing, demilitarisation and the stability of the institutions, as the key to progress. "If we do not get any progress on decommissioning, I cannot see us being able to get progress on other issues", he said.
"If they [Sinn Fein] were to help on decommissioning, I could certainly mediate with the British government . . . But if nobody is prepared to move, then it is much more difficult to encourage the British government to move on policing, on demilitarisation, and to encourage David Trimble to stay on."
Mr Ahern said that the negotiations had moved a long way in private towards a resolution of the demilitarisation issue. But there was "no possibility of bringing that issue to a conclusion without a clear position on decommissioning".
Jim Cusack, Security Editor, writes: Senior Irish and British security officials are understood to have heard yesterday that there is little prospect of any further developments on the decommissioning of IRA arms other than the possible continued inspection of dumps. The officials, together with senior police officers, met at Stormont yesterday to review progress so far on the security aspects of the Belfast Agreement.