The Rev Ian Paisley's Democratic Unionists should enter the 21st century and face political reality, Mr Gerry Adams said tonight.
After meetings with US President George Bush's special envoy to Northern Ireland, Ambassador Mr Mitchell Reiss, and Senator Edward Kennedy in Washington, the Sinn Féin leader said the Belfast Agreement could not be put on hold while the DUP struggled to come to terms with change.
The West Belfast MP said: "The impasse at this time in the talks process is the DUP's failure to accept the fundamentals of the Good Friday Agreement and their failure to accept nationalists and republicans on the basis of equality.
"In particular, they have targeted the power sharing and all-Ireland principles of the Agreement.
"The core issues were negotiated and agreed in the Good Friday Agreement. There will be no going back on these positions.
"That is the new political reality which the DUP has to come to terms with.
The pro-Agreement majority cannot be expected to stand still while the DUP edge their way into the 21st century.
"The DUP's refusal to engage and to accept the reality of the Good Friday Agreement cannot be allowed to paralyse the process of change."
Mr Adams was commenting as British and Irish officials worked on proposals which will be put to the Northern Ireland parties to attempt to break the political deadlock.
The DUP also came under attack tonight from SDLP Assembly member Mr Dominic Bradley who claimed the party was not matching its rhetoric about power sharing in talks or in local councils.
The Newry and Armagh MLA said: "Peter Robinson says they want power sharing. So why doesn't he share power in Castlereagh Council?
"Why won't Paisley agree to power sharing in Ballymena?
"Nigel Dodds tells us they want accountability and collectivity too. So why won't they agree to the 13 separate proposals on collectivity and accountability we put to them?
"Why did they go further and insist on a power to be able to veto any nationalist minister on any issue at any time? That is something that we will never agree to," he said.