Mr Gerry Adams has said the Ulster Unionist leadership must embrace change and lead from the front if the Mitchell review, which resumes today, is to succeed.
Speaking at Sinn Fein's national women's conference in Dublin on Saturday, he said republicans should not underestimate the challenges facing unionists.
"Some of them have faced up to these challenges with courage. Unionism is no longer a monolith. There are people there, there are leaders there, who want to set aside the old ways. We need to work with them to make this happen.
"The biggest problem is that unionists fear change, try to minimise change and see it as being to their disadvantage. This places a huge burden on any unionist leadership that wants to plot a course into the future.
"First of all, the leadership itself has to be for change. It has to be prepared to give political leadership in a way that is totally different from the leadership of the past."
Mr Adams said Sinn Fein had stretched its constituency to the limit and he was disappointed a deal had not been reached at Stormont last week.
"Maybe the UUP were only negotiating. Maybe they just don't know when to stop saying `No'. Maybe it is just too big a jump for them to take. Maybe the real and genuine difficulties they face cannot be overcome," Mr Adams said.
"Maybe they aren't up to it. It is hard to know. We know that leadership has to lead. Let us hope the unionist party leadership will do just that. We want a just and lasting peace.
"Hopefully, the reflections by unionism will have led them to the only possible conclusion if they really want this process to work. And that is that the only way forward is to work with the rest of us in shaping a new future and a new beginning for all sections of our people."
Mr Adams said republicans knew compromises had been made, and it would not be easy for Sinn Fein to enter government.
"They know how big a sacrifice it will be for Bairbre de Brun and Martin McGuinness to take their places as ministers alongside parties that include members who don't want to see a Fenian about the place, never mind a Fenian woman."
About 80 delegates attended the conference, which was addressed by women from the Garvaghy Road and the lower Ormeau Road.
Ms Tina Tully of Sinn Fein Connacht/Ulster said while her party had a better record on female participation than most there was still room for improvement.