Democratic Unionist leader Peter Robinson has resumed his duties as Stormont First Minister and his resigned ministers have returned to office following the publication of a Government-ordered review of paramilitary activity in Northern Ireland.
The DUP last month disengaged with the power-sharing administration in Belfast amid a crisis sparked by a murder linked to the IRA.
The independent assessment of paramilitary structures, which was commissioned as Stormont lurched toward collapse, found that all the main republican and loyalist groups, including the Provisional IRA, still exist; that members have committed murders since the ceasefires of the 1990s; but that their leaders are now committed to peace.
While Mr Robinson said his party was returning to the Executive, he warned that Stormont's problems were not over and insisted on-going political talks aimed at resolving a raft of disputes besetting powersharing only had two weeks to find resolution.
"We really are only back for a couple of weeks while the talks process is concluded because unless we can resolve all of these outstanding issues we won't be able to have a sustainable Assembly and Executive," he said.
The DUP leader added: "We are at the sparring stage still but I think today marks the beginning of intensive discussions."
The review, ordered after the shooting of ex-IRA man Kevin McGuigan in August, said an IRA "Provisional Army Council" remains in place, and IRA members believe that ruling body "oversees" Sinn Fein's strategy.
Stormont's Deputy First Minister Martin McGuinness rejected those claims, insisting his party was the "only organisation" that represented the mainstream republican movement.
"As far as I am concerned Sinn Fein is the only republican organisation involved in the peace process, in democratic politics and in political activism," he said. "We take instructions from no one else."
Mr Robinson stood aside from his duties last month at the height of the political crisis, and three of the party's four other Executive ministers resigned.
They walked away from government when Sinn Fein's northern chairman Bobby Storey was arrested by detectives investigating the McGuigan murder. Mr Storey, who denied involvement, was later released without charge.
The party subsequently employed a controversial tactic of reappointing its ministers for a number of hours and then resigning them again, in a repeating cycle, to prevent the jobs being reallocated to other parties.
The number of DUP ministers out of office increased from three to four during this period, as the party took over a post vacated when the Ulster Unionists - a minor coalition partner - walked out of the administration outright.
Mr Robinson said the independent report had confirmed the assessment of Northern Ireland's police chief George Hamilton in August that individual members of the IRA were involved in the McGuigan murder but the leadership did not sanction it and was wedded to peace.
The party leader suggested the picture painted by Mr Hamilton was subsequently confused by the arrest of a senior Sinn Fein figure - Mr Storey.
He said the report had confirmed Mr Hamilton's view, rather than the "contradictory" implications that flowed from the arrest of Mr Storey.
On that basis, he said, he and his ministers were returning to their ministerial duties. The party policy of boycotting many Assembly sittings is also set to end.
"The review indicates that the leadership of the republican movement is committed to the peace process and is encouraging others to do so," said Mr Robinson.
"But it shows a very unpleasant situation in terms of the reach of the authority of that leadership in that the report shows there are elements of the organisation that are involved in all sorts of illegal activity and that just can't be allowed to continue."
The DUP said it was "disturbed but not surprised" that an IRA Army Council still existed and said the retention of any paramilitary structure, loyalist or republican, was "unacceptable".