Mr David Trimble and Mr Gerry Adams were continuing a series of face-to-face meetings at Stormont late last night and are expected to resume this morning.
They held several meetings yesterday, trying to put together what a senior talks source described as a package of reciprocal confidence-building steps which would allow the Northern executive to be formed and decommissioning of arms to take place.
They are striving to arrange a sequence of reciprocal moves that would ensure the success of the Mitchell review and no loss of face for either republicans or the Ulster Unionist Party, according to senior talks sources.
The success of the review hinges on Senator George Mitchell and the central negotiators for the UUP Mr Trimble and Sir Reg Empey, and for Sinn Fein Mr Gerry Adams and Mr Martin McGuinness.
"There will be no executive on Monday," said one senior talks insider, who added that the parties were trying to create a sequence of events that would lock both sides into the process.
This, he suggested, could involve the IRA appointing an interlocutor to deal with Gen John de Chastelain's decommissioning body, then establishing how disarmament might take place (the modalities), then agreeing with the general a time-frame for disarmament, finally leading to the completion of some form of decommissioning.
At the same time and in a mutual manner the UUP could agree to the nomination of ministers, to the creation of a shadow executive, to the creation of a formal executive, to the creation of the institutions flowing from the Belfast Agreement which includes the North-South bodies so important to republicans.
There is no agreement yet on how the particular sequence might flow, or whether in such an incremental process it would allow the executive to be formed before decommissioning began.
If this reciprocal step-by-step concept was accepted it could allow Mr Trimble argue to his party, and to his grassroots, that the "process" of IRA disarmament had started.
It might also allow republicans overcome their opposition to the idea of a commitment to disarm which they describe as equivalent to surrender.
Mr Adams and Mr McGuinness could contend that this concept did not actually involve a commitment, but rather was part of a process where the Belfast Agreement was being implemented.
Sinn Fein has consistently intimated that if the agreement was implemented some form of IRA disarmament would almost certainly follow.
If such a form of sequencing broke the deadlock it is not clear how long it would take before the executive was up and running, or when decommissioning might begin. But the incremental nature of the idea would seem to conform with Mr Trimble's comments yesterday that "sooner or later this will succeed".
While it is far from certain whether a deal will be done - a Dublin source rated the prospects as "50-50" last night.
It is expected these talks will run at least into tomorrow, and possibly slightly beyond, notwithstanding Senator Mitchell's determination to speedily conclude his review.
Politicians engaged in the review said yesterday that so far Senator Mitchell has not put forward any of his own proposals or suggestions on how a deal might be achieved. "That will only happen when Senator Mitchell knows that Sinn Fein and the Ulster Unionists will accept it," said a Dublin observer.
A leading Unionist hardliner Mr Jeffrey Donaldson last night said: "I think people are fearful about what the outcome of these negotiations might be but I am patient. Let's wait and see what report we get at the party executive [due to meet today]."
