The Sinn Fein president, Mr Gerry Adams, has called on Mr David Trimble to remain "truthful and true to the Good Friday agreement" at next month's meeting of the Ulster Unionist Council.
Republicans had "stretched" themselves to meet unionists' demands on decommissioning and devolution, Mr Adams said.
He was speaking at Belfast International Airport before leaving for a visit to the United States, which will include a meeting with President Clinton.
"We have at considerable risk to ourselves sought to accommodate and to take on board the difficulties of others, especially the Ulster Unionist Party, because that is the imperative of making peace," he said.
"It is now up to David Trimble to lead his party, through the Ulster Unionist Council [meeting] and other party forums.
"It is up to him to be truthful and true to the Good Friday agreement and to do what needs to be done."
Mr Adams said the decommissioning issue was "where it belongs and where it should have been over almost two years ago - with the decommissioning body".
The Sinn Fein leader also criticised remarks by the Northern Secretary, Mr Peter Mandelson, in which Mr Mandelson indicated that he would consult the RUC before making any announcement on demilitarisation.
He accused Mr Mandelson of abdicating his responsibilities under the Good Friday agreement to begin demilitarisation.
"This is one of the issues I am going to be raising [in the United States], the whole slowness, reluctance and refusal to demilitarise, the continued existence of the RUC, the failure thus far to implement the Patten report and the disgraceful decisions around Rosemary Nelson and the failure to move on Pat Finucane's case or the case of Robert Hamill."
Mr Adams also expressed sympathy for the family of Richard Jameson, who was killed in Portadown on Monday night.
"I did not know the man who was killed. I obviously feel for his family, and all of this should be an added incentive on politicians to move ahead and not to create any vacuum which will allow these people [who carried out the attack] to prosper."
The Sinn Fein president said he did not see any reason why the new power-sharing Executive at Stormont would not survive beyond the Ulster Unionist Council meeting in February.
"Anyone that jumps ship at this time is going to drown and I do not see any reason why, having come so far, we cannot redouble our efforts to make this work.
"We have a long way to go. There will be lots of difficulties. There will be lots of crises but I think with the will we can make it work."