Sinn Fein has said Senator George Mitchell's review of the implementation of the Belfast Agreement has reached a critical stage but has refused to give any more details of negotiations.
Last night at Stormont, Sinn Fein vice-president, Mr Pat Doherty, said: "There is an intense discussion taking place, but the very fact that we are still here shows that we have not yet resolved the issues.
"These talks are very serious and they are indeed at a very, very serious juncture.
"Sinn Fein will do all it can to resolve this review and see it comes to a successful conclusion." It is understood that Senator Mitchell, who returned to Belfast yesterday, may end his review this weekend, possibly on Saturday.
A spokesman said: "Senator Mitchell has said from the outset that he hoped the review could be conducted as quickly as possible. He has never alluded publicly to any timeframe or deadline by which it would be concluded."
Sources stressed that while it was unlikely Senator Mitchell would let things continue much longer, he would probably be flexible if it looked likely that a deal could be reached soon.
Senator Mitchell held individual meetings with the UUP, Sinn Fein and the SDLP.
He had returned to Belfast with proposals for ending the political deadlock. It is not known whether he gave the parties a document containing his suggestions.
Before they met Senator Mitchell, the UUP, the SDLP and Sinn Fein held an hour-long discussion. They were then joined by the Alliance Party and the Women's Coalition.
The SDLP deputy leader, Mr Seamus Mallon, said he sensed progress had been made. He said it would be known this week if it was enough to reach a compromise agreement.
"There is some movement. There's no doubt about that. Is the movement sufficient to allow both sides to move into a position which will allow the entire process to proceed? That is the key question, and we should know the answer to that very shortly, and I mean this week."
Ms Monica McWilliams, of the Women's Coalition, said she detected some progress.
"David Trimble's problems are very public. Gerry Adams's problems are very private. When you share those kinds of problems together, then there is definitely movement."
Pro-agreement campaigners picketed outside the talks. They set up an ironing board under the banner "Iron out your differences."