Peace process gains in Northern Ireland cannot be taken for granted and much work is needed to address the unresolved issues facing the region, a former US diplomat said as he arrived in Belfast to chair a new political talks initiative.
Dr Richard Haass has begun the first round of negotiations aimed at resolving three of the most divisive problems facing the power sharing institutions at Stormont.
Supported by US foreign affairs expert Dr Meghan O'Sullivan, he is attempting to find consensus on the contentious matters of flags and emblems; parades; and dealing with the legacy of the past.
The ex-White House special envoy acknowledged that a troubled summer in Northern Ireland, when simmering community tensions boiled over into street disorder on a number of occasions, was indicative of the urgency around finding an agreed way forward.
"There's been tremendous progress but, that said, there is still a real need to move things forward and that is again why we are here," he said.
"I think this last summer was something of an indication or something of a warning that one should not take the improvements for granted.
"One has to embed it and one has also to broaden it and there's obviously unresolved issues and unresolved tensions or again you wouldn't have had the violence you had this summer and you wouldn't have had these lingering and persistent political differences and I think the five parties recognise that."
He will meet with them individually in the coming days before holding plenary talks at the end of the week.
Dr Haass will also meet senior clergy and business figures as well as representatives from some of the smaller political parties during a week-long series of engagements.
Another round of negotiations is expected next month with further substantive talks in November ahead of the December deadline for recommendations.
The first party to meet Dr Haass at Belfast's Europa Hotel was Sinn Fein, with SDLP representatives following later in the day.
Unionist parties are due to meet Dr Haass, the current president of US think-tank the Council for Foreign Relations, in the coming days.
A published author on the art of negotiation, Dr Haass said the agenda and timetable was "ambitious but possible".
"We look forward to all sorts of give and take in these days and obviously subsequent visits," he said.
"The goal here is to come up with a consensus document that ideally would be both broad and deep, dealing with these three sets of issues, and that is our goal at this point."
The Haass talks are being billed as the most important since the Hillsborough negotiations when responsibilities for policing and justice were devolved to the power-sharing Stormont Executive.
They follow one of the most difficult summers in Northern Ireland since the signing of the Good Friday Agreement in 1998.
Loyalist tensions over flags and parades sparked mass protests, some of which boiled over into violence on the streets while republicans were heavily criticised by victims campaigners for holding an IRA commemoration parade in Castlederg, Co Tyrone - a town which suffered significantly at the hands of paramilitaries during the Troubles.
At the Executive, political relations between the Democratic Unionists and Sinn Fein have also been strained and last month the DUP withdrew its support for a controversial peace centre on the site of the former paramilitary prison at the Maze in Co Antrim.
Meanwhile, the threat from dissident republican terror groups opposed to the peace process continues to remain severe.
After meeting with Dr Haass today in an encounter that lasted just over an hour, Mr McGuinness emerged with the rest of the Sinn Fein delegation and expressed optimism that resolution could be obtained.
"We are approaching all of this in problem-solving mode," he said.
"These are very, very serious issues that badly need resolution and we are very determined to play a positive and constructive role during the course of these discussions to find a resolution to these problems, because quite honestly that's what people want - they want to see us moving forward decisively, building a better society, a new future for our young people, bringing inward investment, creating jobs and taking advantage of whatever tools at our disposal to ensure that people have a better standard of living."
The Deputy First Minister added: "If there's a will and if there is a determination and if there is a generosity, yes, these issues are resolvable.
"I think we are all agreed that dealing with the issue of the past might be the most difficult of all, but I think if people come at this with a good heart, huge progress can be made."