"There's obviously unresolved tensions, or you wouldn't have had the violence you had this summer and you wouldn't have had these lingering and persistent political differences," Haass told reporters before meeting Catholic politicians.
Both sides' leaders agree they need help identifying solutions able to ease tensions in a year that has already featured two protracted periods of rioting. Nobody has been killed, but more than 100 police officers have been injured in clashes chiefly with militants from the province's Protestant majority.
The Irish nationalist Sinn Fein party, which represents most Catholics in the British territory, was the first to meet Haass. He plans to meet Protestant leaders later in the week.
Sinn Fein's Martin McGuinness, who is deputy leader of the five-party government in Northern Ireland, said he believed Haass could identify solutions that both sides should be able to accept "if there's a will and determination and generosity."
Predominantly Protestant parts of Belfast suffered street clashes from early December to mid-February after Catholic politicians on Belfast City Council voted to curtail the flying of the British flag at Belfast City Hall.
Protestants demanding a reversal of the decision mounted nightly illegal road blockades that often degenerated into running battles with riot police.
Protestant extremists again mounted several nights of attacks on police in July after British authorities barred an annual parade by the Orange Order brotherhood from passing, as it usually does, a hard-line Catholic district called Ardoyne. Catholics generally loathe the Orangemen's July parades and want them blocked from passing near Catholic districts.
The Good Friday accord paved the way for major peacemaking achievements, including sweeping police reform designed to boost Catholic support, British military withdrawals, disarmament of major paramilitary groups, and the formation of a Catholic-Protestant government in 2007.
The main faction of the Irish Republican Army, the Provisional IRA, killed nearly 1,800 people during a failed 1970-1997 campaign to force Northern Ireland out of the United Kingdom. The Provisionals renounced violence and surrendered weapons in 2005, but small IRA splinter groups still attempt occasional bomb and gun attacks in hopes of undermining the unity government.