DUP leader Arlene Foster has accused Sinn Féin of shattering trust in efforts to restore powersharing.
Accusing the republican party of leaking confidential papers in the aftermath of a Valentine's Day talks bust-up, Mrs Foster claimed Sinn Féin 's behaviour was unlike anything she had ever witnessed in her political career.
In a stinging attack against her erstwhile partners-in-government, the former Stormont first minister said Sinn Féin negotiators now needed to prove they could be trusted again.
"They have behaved in an incredibly bad way, therefore the building up of trust is going to take a long time and it is going to take actions," she said.
"We have heard a lot from Sinn Féin in relation to reaching out, it's about time that they recognised the role they had in relation to the breakdown and in relation to the shattering of trust within the unionist community.
"I think Sinn Féin has to look at themselves and did they make the right decisions to act in the way that they did in February past, because that has caused a great schism in relation to the politics here in Northern Ireland."
Mrs Foster launched the broadside against Sinn Féin after meeting with Northern Ireland Secretary Karen Bradley at Stormont.
Mrs Bradley met all the North's main parties today as she continues to consider ways to revive devolution.
Northern Ireland has been without a properly functioning powersharing government for almost 16 months, due to the bitter standoff between the two largest parties, the DUP and Sinn Féin.
A row that broke out over a botched green energy scheme, and widened to encompass long-standing disputes such as the Irish language and gay marriage, shows no sign of resolution.
After negotiations collapsed in acrimonious circumstances in February, the parties traded claim and counter-claim about whether a proposed deal had been in the offing.
Sinn Féin claimed Mrs Foster had signed off on a deal before backing out in the face of an internal party revolt - claims the DUP leader vehemently denied.
Amid the fallout, documents exchanged by the parties during negotiations were leaked to sections of the media - incidents the DUP has firmly blamed on Sinn Féin.
"Their behaviour after the breakdown of talks in February was quite disgraceful, quite disgraceful," Mrs Foster said today.
"It has never happened before in any talks process I had been involved in and I have been around in a lot of talks processes.
"They gave out position papers, tried to sell them as the agreement and frankly the fact there was no agreement was the reason the talks broke down."
Earlier, Sinn Féin accused the DUP of "checking out" of powersharing and making no effort to find a way to restore devolution.
The party's vice-president, Michelle O'Neill, also claimed the UK Government's confidence and supply deal with the DUP at Westminster was now the "greatest obstacle" preventing the resurrection of coalition government in Belfast.
She heavily criticised the DUP and Conservatives after holding what she described as a "frank" meeting with Mrs Bradley.
Mrs O'Neill said that while she had met Mrs Foster since February, at events to which they had both been invited, she insisted there had been no "meaningful" engagement in the last two-and-a-half months.
"We haven't had a real or meaningful engagement with the DUP because they have checked out," she said.
"Since the talks collapsed the DUP have been preoccupied by Brexit, they have been preoccupied with their relationship with the Tories at Westminster and they are not engaged in terms of trying to get these institutions up and running again.
"I don't think they should get carried away with their supply and confidence deal, which we all know will be shortlived.
"The effort should be here, it should be on negotiations, it should be on getting these institutions up and running and functioning for all people."
Mrs O'Neill said today's meeting represented the first real attempt by Mrs Bradley to engage with the parties in two-and-a-half months.
"I put this directly to her - I believe the British Government are prioritising their supply and confidence deal over getting the institutions up and running here in the North," said Mrs O'Neill.
"And therein lies the greatest obstacle to getting the institutions up and running and an executive that's functioning for all of our citizens.
"The DUP have checked out of powersharing."
She added: "That is the wrong approach and that is an approach that is being pandered to by the British Government because of their supply and confidence deal."
Six weeks ago, Mrs Bradley signalled an intent to cut the salary of members of the crisis-hit Stormont Assembly.
She said she was "minded" to slash MLA pay by 27.5%, but would seek the views of the local parties before making a final decision.
The Conservative MP has still not indicated what her final decision will be.
Mrs O'Neill said she told the Northern Ireland Secretary to get on and implement the cut.