The Taoiseach (Irish Prime Minister), Enda Kenny, has called Theresa May to express his concern over a possible deal between her party and the DUP.
He sought assurances that "nothing should happen to put the Good Friday Agreement at risk".
A spokesperson from Number 10 said both leaders spoke on the phone on Sunday.
They said Mrs May and Mr Kenny had "confirmed their joint commitment to restoring the Northern Ireland Executive as soon as possible".
"The Prime Minister reiterated that the government's approach and objectives in the forthcoming talks to re-establish the Northern Ireland Executive remained unchanged," they said.
The DUP leader Arlene Foster said good progress had been made in talks with the Conservatives, but a deal is yet to be reached.
Earlier, some of Northern Ireland's other parties expressed concern that a deal between the Conservatives and the DUP could hamper power sharing at Stormont.
Talks aimed at restoring the executive resume on Monday and all five main parties have said they will be there.Image copyright PACEMAKER Image caption The five main Northern Ireland parties have said they will resume talks on Monday
The DUP has indicated its potential role at Westminster does not change its desire to restore power sharing.
They say they remain ready to form an executive without preconditions.
But the other Stormont parties have warned that any deal between the DUP and Conservatives could make restoring power sharing more difficult.
Devolved government in Northern Ireland broke down in January.
The late deputy first minister, Martin McGuinness, stood down to protest the DUP's handling of an energy scandal, in a move that triggered a snap election.Image caption Sinn Féin want a new chairperson for the talks at Stormont
Former Sinn Féin finance minister Máirtín Ó Muilleoir said the government would now be "dancing to the DUP's tune".
SDLP MLA Nichola Mallon also said Mr Brokenshire could not be considered "an honest broker".
She said the situation had been difficult before, but had got even worse.
"How can you have a secretary of state sitting as an honest broker when they already have a deal with one of the parties sitting around the table?" she asked.
The Alliance Party's Stephen Farry said: "There's now a massive problem, he cannot be an impartial broker.Image caption Stephen Farry has also called for a new chair for the talks
"The government will have one hand behind its back and if it tells the DUP to agree to something it doesn't want to then the DUP will pull the plug and the whole thing will come crashing down.
"We cannot stop them from making this deal, but the repercussions for Northern Ireland are potentially severe."
The UUP's Danny Kennedy said the other parties would have to "wake up" and accept the situation.
"The UUP will be at Stormont tomorrow," he said.Image caption Danny Kennedy said the UUP would be at the talks on Monday
"That's what people want.
"The secretary of state and indeed the prime minister have made no secret of their own unionist views. They support the union and we welcome that."
Mrs Foster and her ten DUP MPs are due to meet with Theresa May on Tuesday.
No one from the DUP was available to appear on the Sunday Politics programme.