The prime minister has broken his pledges over IRA disarmament, DUP leader Ian Paisley has said.
Mr Paisley said Mr Blair must 'keep his promise'
Mr Paisley was speaking on Thursday after a second day of talks at Stormont which have been concentrating on issues arising out of a review of the Good Friday Agreement.
The talks are aimed at paving the way for more intensive negotiations chaired by the British and Irish prime ministers later this month.
Northern Ireland Secretary Paul Murphy said the peace process was reaching a point of decision.
Mr Murphy said the present talks were "very serious, important and crucial" - and were capable of achieving a resolution.
He said the point of the talks was to implement the Agreement, and rejected any notion the government was prepared to let go of the fundamentals of the deal.
Irish Foreign Minister Brian Cowen said no-one was seeking to fudge the issues, and agreed a resolution was possible on issues such as the IRA, decommissioning and policing.
Paul Murphy hosted the talks at Stormont
Mr Cowen said the political process was at a "critical" point and the parties would need to use the next two weeks to prepare themselves.
Mr Murphy and Mr Cowen are also expected to host more talks next week.
At a news conference after Thursday's talks, Mr Paisley demanded that Mr Blair must "keep his promise" about bringing an end to the IRA and decommissioning all weapons.
Mr Paisley said the prime minister "carried the burden of guilt on his shoulder" and promised to "nail" Mr Blair on the issue at the intensive talks later this month.
BBC Northern Ireland political correspondent Martina Purdy said Mr Paisley appeared to give out unclear messages on whether he would deal with Sinn Fein if the IRA both decommissioned and disbanded.
At one point, Mr Paisley said Sinn Fein would have to become a new party. He also said Sinn Fein should not be at the negotiating table while the IRA existed and remained armed.
Mr Paisley said this was a matter which Mr Blair must address. He added that "there is going to be no sell out".
Sinn Fein's chief negotiator Martin McGuinness said he was "looking forward" to going into government with the DUP.
Mr McGuinness told a news conference at Stormont that his party was determined to "do the business" at the intensive talks later this month.
He said they had been told in "private discussions" that the DUP wanted to make a deal, and that they had to recognise the new political reality.
"If they want to be in government, they are only going to be in government with Sinn Fein," he added.
Senior Ulster Unionist Sir Reg Empey warned that north-south bodies could not continue indefinitely if there was no return to devolution. He said the bodies were inextricably linked with the assembly.
Sir Reg said the government must "clear up confusion" about the purpose of the forthcoming intensive talks.
He asked whether the aim of the talks was to tweak the Agreement or to forge a new one.
SDLP leader Mark Durkan said devolution could only be restored in a "confident and lasting manner" by "full and final resolution of the two crux issues which caused suspension, ongoing paramilitary activity and failure to work the power sharing institutions".
The governments hope the negotiations, believed to be planned for Leeds Castle in Kent later this month, will pave the way for a return to devolution.
The political institutions in the province were suspended in October 2002 amid allegations of IRA intelligence gathering at the Northern Ireland Office.
A spokesman for Mr Blair said on Tuesday he believed there was a "shared agenda" between the parties which could lead to a deal.
The Ulster Unionist leader, David Trimble, has not been at Stormont for the talks as he is at the Republican Party convention in New York.
The Ulster Unionists withdrew from the review of the Agreement, saying the focus should be on ending paramilitarism, but they have now indicated they will take a full part in the main talks.