Sinn Fein president Gerry Adams told supporters in the US that he wants to reach an accommodation with unionism, but hit out at what he called its “Afrikaner wing”.
The republican leader told a fundraising dinner for his party in New York that partnership was the way forward, but that some unionists opposed change.
The DUP branded his comments offensive.
This came as the dispute between republicans and the DUP yesterday forced the cancellation of a further meeting of the Northern Ireland Executive, which has not sat since June.
Mr Adams said: “Once, unionism dominated life in the north... today, if political unionism wants to exercise power, it can only do so if it is prepared to work in partnership with republicans and within the all-Ireland political architecture of the Good Friday and St Andrews Agreements.
“Some elements of unionism continue to resist change. But for the leadership of unionism today the question is very simple.
“Is it ready to move forward with the rest of us?”
Sinn Fein and the DUP are in talks aimed at ending deadlock over issues including the devolution of policing and justice powers from Westminster.
“Few human beings of my acquaintance are as petty and mean-spirited and negative as those in the Afrikaner wing of unionism,” he said.
“But if we are truly about nation-building — and that is what Sinn Fein is about — then we cannot allow ourselves to be distracted or diverted by negative and reactionary elements.”
He said the political institutions needed to be bedded down, and outstanding issues needed to be dealt with to fulfil the promise of the Good Friday Agreement.
Sinn Fein has blocked Executive meetings since June in protest at the refusal of the DUP to accept devolution of policing and justice powers from Westminster until it believes the unionist community is ready for the move.
Republicans claim that the DUP is failing to adhere to the partnership principles of power-sharing and has accused the DUP leadership of looking over its shoulder at right-wing elements.
Deputy leader of the SDLP Alasdair McDonnell said the failure to hold Cabinet meetings was hurting ordinary people.
“There can be little doubt that 2008 will forever be remembered as the year the credit crunch bit, the economy slid and Sinn Fein and the DUP did absolutely nothing,” he said.
But DUP Assembly member Robin Newton attacked Mr Adams for comparing sections of unionism to white supremacists in South Africa.
“This hopelessly sectarian outburst, which bears no semblance [sic] to reality, says more about Mr Adams than it does about any unionist,” he said.
“It is clear that there is an underlying bitterness in these disgraceful comments.
“It would have been more appropriate for Mr Adams to tell his American audience that it is his party which is in breach of its legal, political and moral responsibilities by stopping devolution from functioning in the fullest and most effective way.
“Furthermore, I wonder, did Mr Adams tell his American friends that it was his party which fought against and protested at the sight of people honouring our troops returning home from serving alongside our American allies in Afghanistan?”