Talks aimed at resolving the remaining contentious issues of the Northern Ireland peace process have broken up without a deal before Christmas.

The chair, the US diplomat Richard Haas, is flying home without an agreement from the five main political parties sharing power at Stormont.

While the parties talked through the night into Christmas Eve they could not reach a deal although Haas and Harvard professor Meaghan O'Sullivan may return in the new year to host more negotiations.

The talks hinged on a propsal for a new super investigative body that would probe into past Troubles crimes.

But the Guardian has been told that the Ulster Unionists objected to the new policing-the-past unit because ex members of the security forces could be arrested over incidents in the Troubles as well as loyalist and republican paramilitaries.

Haas and O'Sullivan were brought to Northern Ireland in July after the first minister, Peter Robinson, asked them to chair talks over flags, parades and unsolved crimes and other issues connected to 35 years of conflict.

In a tweet this morning Haas said the talks had broken up without agreement but he hoped future discussions would succeeed.

Later outside the Stormont Hotel, Haas said: "I am not in the business of doing post mortems here because the patient is still alive."

The Democratic Unionists, the UUP, Sinn Fein, the SDLP and Alliance discussed four separate plans, each of which were vetoed through days of talks.

DUP Lagan Valley MP Jeffrey Donaldson said: "Obviously I think everyone is disappointed that we are not having an agreement at this stage.

"But we have to be realistic - there remain significant issues of difference across all three areas that were under negotiation.

"I think with further effort we are capable of closing the gap but it just didn't happen tonight."

Sinn Fein's negotiator, Gerry Kelly, said the party's team had been mandated by its ruling executive to try to secure agreement in the meeting.

"We are disappointed that we weren't able to do that," he said.

"Probably more important is that people watching this will be more disappointed if we can't bring this to some sort of conclusion in the few next few days and phases.

"When it came to a deal, I think we could have done it, (but) we didn't manage to do it."