Talks led by former US diplomat Richard Haass aimed at ending disputes on flashpoint issues hampering peace in Northern Ireland broke up Tuesday without agreement.
Haass was called on in September to help the main political parties end arguments over flags and parades which have caused rioting in the British province.
The disputes are the legacy of the Troubles, the three decades of sectarian unrest between pro-British Protestants and Republican Catholics that largely ended in 1998.
"It would have been nice to come out here tonight and say we have got all five parties completely signed on to the text -- we are not there," Haass said after the talks wrapped up.
The parties had set a deadline of December 31 for an agreement.
Haas, the former US envoy to the province, said it was "no secret" that the issue of national flags was the "toughest" to resolve
But he said there had been "significant progress" all round.
Outbreaks of rioting over the past year were the worst in Northern Ireland for years as community tensions over the marching season in the summer, when republicans and unionists hold parades to mark historic dates, spilled out on to the streets.
Violent protests also took place in December 2012 over a decision by Belfast City Council to restrict the number of days that the British flag was flown at City Hall.
British Prime Minister David Cameron said it was "disappointing" not to reach a full agreement but said the talks had "achieved much common ground".
"I urge the parties to keep going. I also want to thank Dr Richard Haass and his team for their dedicated work," he said in a statement.
Haass's role follows a tradition of US involvement in Northern Ireland, after former US senator George Mitchell won widespread praise for chairing the negotiations that led to the 1998 Good Friday Agreement.
That deal largely brought an end to the Troubles although sporadic attacks and violence continue with a series of bombings and attacks in recent weeks.