Many unionist leaders have "abdicated responsibility" for weekend violence, President George Bush's special envoy to Northern Ireland has said.
US special envoy Mitchell Reiss said he was 'disappointed'
Mitchell Reiss said leadership was needed but "in the last few days we haven't seen very much of it".
DUP leader Ian Paisley denied prompting riots by saying the parade re-routing "could be the spark which kindles a fire there would be no putting out".
Mr Paisley condemned the violence but said his prediction had come true.
"I was telling the truth, I said I was very very worried," he said on Monday.
"At that time I was in the midst of trying to get a way whereby this would not happen. And it has happened - my words have been proved to be right."
Two nights of violence began on Saturday when a controversial Protestant Orange Order march was re-routed away from the mainly Catholic Springfield Road area of west Belfast.
After a request by unionists on Friday, the Parades Commission reviewed its ruling on the route, but decided not to change it.
In a BBC interview, Mr Reiss said there was "absolutely no excuse" for the trouble.
"I think all of us are pretty disappointed with the abdication of responsibility by many unionist political leaders," he said.
"No political party, and certainly no responsible political leadership, deserves to serve in a government unless it cooperates and supports fully and unconditionally the police, and calls on its supporters to do so.
"It's true for unionism, it's true for all political parties, and I think that this was not the finest moment for politics in Northern Ireland over the weekend."
The US Envoy said problems needed to be tackled by sustained hard work in communities.
"What you really need is leadership, and unfortunately in the last few days, we haven't seen very much of it," he said.
However, he singled out Ulster Unionist Belfast councillor Fred Cobain for praise for the work he had done over the weekend and in the past weeks.
"When people do stand up and take a courageous stand and exert leadership, they deserve to be recognised," he said.
Irish prime minister Bertie Ahern said that whatever grievances people may have, there was "absolutely no justification for violence". He has asked for a full report from the Joint Secretariat in Belfast.
Sinn Fein president Gerry Adams accused Mr Paisley and Ulster Unionist leader Sir Reg Empey of giving "wrong and negative leadership". He said they could not wash their hands of what happened.
Mr Adams said if he had said "even a measure" of some of the comments made prior to the march, there would have been calls for his arrest.
Policing Board vice chairman Denis Bradley said "irresponsible" comments by DUP politicians were not helping the situation.
He said loyalists must realise rioting was "not the answer".