President Bush's special envoy to Ireland, Mr Mitchell Reiss, and Sinn Féin's Mr Martin McGuinness have engaged in a strongly-worded clash of views ahead of tomorrow's peace process visit to Hillsborough Castle by the Taoiseach, Mr Ahern, and British Prime Minister, Mr Tony Blair.
Sinn Féin was stung by Mr Reiss's weekend comments that the party had told "massive untruths" about police reform in Northern Ireland. Mr Reiss depicted as false the recent Sinn Féin advertisement in the New York Times condemning the Police Service of Northern Ireland (PSNI) as deeply flawed.
Mr Reiss, following on similar exhortations from Republican and Democratic politicians in the US last week, called on Sinn Féin to join the Policing Board and was heavily critical of the $25,000 Sinn Féin advertisement.
"At best it was enormously misleading, at worst it was untruthful. I'd be happy to go down, point by point, with a rebuttal to each of the allegations or justifications that Sinn Féin had for not joining the Policing Board," said Mr Reiss.
"But the bottom line is there's massive untruths there and they need to reconsider their position as soon as possible," Mr Reiss told BBC Radio Ulster's Inside Politics programme on Saturday.
Mr Reiss said the Sinn Féin president Mr Gerry Adams and the party's chief negotiator could best improve policing by nominating members to the Policing Board.
He said that the DUP was wrong not to talk to Sinn Féin but by the same token Mr Adams was wrong not to talk to the PSNI chief constable Mr Hugh Orde.
"It's hard to see what grounds Sinn Féin can give for not talking to the chief constable," he added.
The harsh tone of Mr Reiss's comments appeared to catch Sinn Féin by surprise. Mr McGuinness said he was baffled by the US diplomat's remarks and would raise his concerns about the comments directly with Mr Reiss.
"I spent an hour speaking with Ambassador Reiss in Washington on Tuesday. I find it strange he did not raise his criticisms with me at that time," he added.
Sinn Féin would not join the Policing Board until it was fully satisfied about police reform, he said.
British and Irish sources said at the weekend that Mr Ahern and Mr Blair at tomorrow's Hillsborough talks would be largely focused on trying to persuade Sinn Féin to use its influence to persuade the IRA to end all activity.
The two leaders will engage in trilateral talks with the parties tomorrow afternoon.
There would be a dual concentration on ending paramilitarism and seeking unionist assurances that they would share power with Sinn Féin if the IRA ended activity, but primarily what was required to restore devolution was a cessation of all forms of IRA violence, the sources said.
Comments from senior Sinn Féin figures indicated that they expected to come under pressure from the two leaders tomorrow, and that they would strive to put the onus for movement back into the responsibility of the British and Irish governments or else claim the party was being unfairly "demonised".
Mr McGuinness said, "The failure of the Irish and British governments to act on their commitments is the main impediment to progress. Their failure to deliver on policing, justice, human rights and equality have undermined the potential for progress."
The Sinn Féin chairman Mr Mitchel McLaughlin complained of attempts by the Irish and British governments, supported by "anti-Sinn Féin elements of the media", to vilify Sinn Féin by making various allegations against republicans.
