* Province has witnessed one of worst years of violence since 1998 peace
* U.S. chairman to leave after six months of talks
* He proposed document as basis for agreement
* Pro-British unionist party 'profoundly disagrees' with parts of text
By Ian Graham
BELFAST, Dec 31 (Reuters) - Marathon talks between the leaders of Northern Ireland's Catholic and Protestant communities broke down on Tuesday without agreement to ease tensions that have led to one of the worst years of rioting in the British province for a decade.
The U.S. diplomat chairing the talks said the five largest parties in Northern Ireland failed to reach an agreement during 18 hours of talks that ended shortly before 0500 GMT, the culmination of six months of negotiations.
No date was set for the resumption of the talks, which were a response to some of the highest levels of street violence and attacks by militant groups since a peace and power-sharing deal in 1998.
That put an end three decades of sustained sectarian violence in the province between pro-British Protestants and Catholics who generally favour unification with Ireland.
"It would have been nice to come out here tonight and say we have all five parties completely signed on to the text. We are not there," said Richard Haass, the president of the U.S. Council on Foreign Relations think-tank and a former adviser to the President George W. Bush on Northern Ireland.
He said he hoped that further talks would lead to some parties signing up to the draft agreement and others would "endorse significant parts of it."
The text proposes the creation of new institutions to deal with contentious parades and the investigations of crimes committed during three decades of sectarian conflict that began in the late 1960s in which more than 3,600 people died.
Haass said the parties had failed to make any significant progress on controversy over the flying of flags in Northern Ireland.
Dozens of police were injured during weeks of rioting early this year after a decision to cut the number of days the British flag flies over Belfast city hall, with officers firing plastic bullets and water cannons.
Several bombs have been planted in central Belfast in recent months by Irish militants opposed to the 1998 peace deal, but none has caused serious injury.
Sinn Fein, the largest Irish nationalist party in Northern Ireland, said it believed the text proposed by Haass provided the basis for an agreement.
The pro-British Democratic Unionist Party, Northern Ireland's largest party, said it would also consider it, but that it "profoundly disagreed" with some of the language in Haass' text.
Haass said he would return to Washington, but held out the possibility of "a limited role" in the future. (Editing by Conor Humphries; Editing by John Stonestreet)