US diplomat Richard Haass believes agreement around Northern Ireland's most divisive issues is in sight – but dealing with the past remains the main obstacle.
Dr Haass said he was more confident of reaching a resolution after concluding four days of critical talks aimed at breaking the deadlock over flags, parades and the past.
He said there was "a real chance to succeed" – but warned that it depended on the willingness of people to make "tough decisions".
Before returning to the United States with foreign affairs expert Prof Meghan O'Sullivan, the vice chair of the talks, Mr Haass said: "Both of us leave here with somewhat more confidence than we had coming in."
Mr Haass revealed that since arriving on Tuesday, they have held 30 meetings and met more than 100 people.
The pair will fly back to the US today but will hold more round-table talks next month.
They hope to have an agreed document by December.
Ms O'Sullivan said there was "a qualitive difference" between the three issues they have been brought in to resolve.
"The past seems to be in a different category," she said.
"That seems to be the assessment not only of ourselves but of virtually everyone we speak with."
First Minister Peter Robinson has said he does not expect that all the issues would be dealt with by the end of December.
However, Mr Haass said he was still committed to reaching a conclusion within the timeframe.
"We have made it clear all along that we are operating under a deadline, that we will finish our work before December 31 of this year, and let me just say that nothing that I've heard and nothing that I've read leads me to think that more time would result in more progress."
He said the "critical variable" was not time, but the willingness and ability of political leaders to make difficult choices.
He continued: "For this round and the next round the thrust of what we are doing is listening and asking and then come November and December the nature of the interaction will change."
Yesterday Mr Haass chaired a plenary session involving the five Executive parties.
The parties agreed their round-table meeting had been constructive.
US diplomat Dr Richard Haass and team of negotiators arrived in Northern Ireland this week to try and reach consensus on the flags, parading and dealing with the past. They used this week for exploratory meetings. Mr Haass and his team return in late October when they are expected to hold meetings in London and Dublin.