Dr Haass, who chaired unsuccessful all-party talks in NI last year, said the region could slip back to the levels of violence seen during the Troubles if political progress isn't made.Speaking in Washington on Thursday, Peter Robinson and Martin McGuinness said the comments had not been helpful.Mr Robinson, DUP leader, said: "I think that he needs to perhaps use more caution when he indicates there could be a further outbreak of violence."That isn't helpful and I think we all know what the repercussions of that can be."Martin McGuinness, of Sinn Féin, said: "Our responsibility as politicians is to stand together against all of those who would wish to plunge us back to the past. I think we have been very successful in doing that over the course of the last number of years."In the last 18 months we faced particular challenges with violence on the streets and police men and women being injured, but I don't have any sense whatsoever that any sizable section of our community would support any group who would try and plunge us back to the past."Dr Haass was left disappointed when months of negotiations failed to result in an agreement on how to deal with controversial and divisive issues like flags, parades and the past.He told the US Congress: "Absent political progress, the passage of time will only create an environment in which social division intensifies, violence increases, investment is scared off, alienation grows, and the best and brightest leave to make their futures elsewhere."Mr Robinson and Mr McGuinness have been in the US on a business mission this week.