Senator George Mitchell last night become embroiled in a controversial exchange with the UK Unionist leader, Mr Robert McCartney.
On the second day of his review of the implementation of the Belfast Agreement, Mr Mitchell met three anti-agreement unionist parties, the UK Unionists, the Northern Ireland Unionist Party and the United Unionist Assembly Party.
The anti-agreement politicians questioned his independence and impartiality. Speaking after his meeting, Mr McCartney described Mr Mitchell as "a dispirited man showing no indication of the slightest optimism".
However, in a statement issued through a spokesman, Mr Mitchell insisted he was not downhearted.
"The senator was in good form all day. But, if there was a moment when he became dispirited, it was probably at the prospect of having to listen to Robert McCartney," the statement said.
Mr McCartney said Mr Mitchell's reply showed his analysis of the former senator's mood was correct. "If ever there was any evidence confirming the accuracy of my observation about Senator Mitchell's demeanour during our meeting, then it was supplied by the personal nature of his response," he said.
The UK Unionist leader questioned Mr Mitchell's credentials to chair the review.
"George Mitchell is not arriving here as a neutral. George Mitchell has taken all the plaudits for this agreement," said Mr McCartney.
"He is depicted as a prince of peace, a peacemaker of international reputation, and when I suggested to him that the whole thing is falling apart because of its inherent dishonesty and defects, and he was here to effect a builder's repair list, he was unable to give any answer."
The Northern Ireland Unionist Party leader, Mr Cedric Wilson, said he had a "stormy" meeting with Mr Mitchell. "It's time for him to go home," he added.
He accused the former senator of being an "enforcement officer" for the agreement, not an independent observer.
"His judgment is clouded to say the least. You cannot have someone reviewing the process who has himself a fixed position - that he believes Sinn Fein must be brought into government."
As the chairman of the talks which led to the agreement, Mr Mitchell was not impartial, Mr Wilson said. His role was like a judge being involved in the retrial of a contentious case he himself had originally ruled upon. Mr Wilson said his party would seek legal advice about Mr Mitchell's position and would consider a judicial review challenging it. However, if Mr Mitchell was truly a man of honour he would realise his position was untenable and leave.
The Ulster Democratic Party, the UDA's political wing, also met Mr Mitchell. The UDP leader, Mr Gary McMichael, said: "The question still to be answered is whether the major parties involved in the review genuinely want to reach a compromise acceptable to the people of Northern Ireland."
He called on the Provisional IRA to reassure unionists that it was committed to exclusively peaceful methods.