Sinn Féin will not be shoe-horned into a power-sharing deal cobbled together by the Government to appease to the DUP, Michelle O'Neill has warned Tories.
Speaking at an event on the fringes of the UK Conservative Party conference alongside her DUP counterpart, Ms O'Neill said the unionist deal to prop up the Conservatives posed "real challenges".
Arlene Foster insisted the electoral pact with the Tories was "not a distraction" from the negotiations in Belfast.
But despite breaking bread together at the Ulster Fry, the leaders clashed over Britishness.
Ms O'Neill told the event organised by peace charity Champ: "The north isn't British."
"I don't want this to turn into a row but Northern Ireland is British," the DUP leader insisted.
The rare joint appearance saw the two women share a table for breakfast before sitting side by side on the stage in the Great Hall in Manchester's Town Hall.
One Tory activist told the pair he hoped they would become known as the "Chuckle Sisters" - a reference to the Chuckle Brothers tag given to the surprisingly warm relationship Ian Paisley and Martin McGuinness developed when they led the power-sharing government.
But the leaders showed no sign of reaching a breakthrough despite both insisting talks had intensified.
Ms O'Neill said: "The British Government should not think that they can cobble together a deal acceptable to the DUP and then shoe-horn Sinn Féin into acquiescing to it.
"That will not happen. The shape of a deal is very clear. The two Governments know this. So do the DUP and the other parties."
Mrs Foster warned that "decision time is soon upon us" as the prospect of a return to direct rule from Westminster loomed large.
The UK Government is warning that it will have to step in to pass a budget for the region's rudderless public services at the end of October.
Ms O'Neill insisted an agreement must be reached on an Irish language Act - the main obstacle in the way of a return to devolved government - but Mrs Foster said any deal must be acceptable to unionists and nationalists.
Asked if she had been too high-handed with Republicans while in office, the DUP leader replied: "Anybody who looks back at their life and doesn't say 'Oh, I should have done that in a different way' would be telling you a lie.
"I don't think I was high-handed but, of course, it's for others to decide if I was or not."
Mrs Foster said "solid progress" had been made but "differences do remain".
"While our electoral strength in the House of Commons is now widely recognised, I have always made it clear that our important role in London will not be a distraction from what we need to do in Belfast," she said.
"It is not a choice for the DUP between influence in London and executive power in Belfast. What is in the best interests of Northern Ireland is operating in tandem and this remains our goal."
The North has been without a first and deputy first minister since January and a functioning executive since March. The institutions collapsed when Mr McGuinness resigned over the DUP's handling of a botched renewable heat scheme, laying the blame at Ms Foster's door.
Pressed on whether the DUP leader must stand aside in order for a deal to be reached, Ms O'Neill said until all outstanding issues had been dealt with, they "will never get to position of Arlene standing aside".
The event was cut short because Ms Foster was heading off to a private meeting with Prime Minister Theresa May.