The Prime Minister has insisted the basis for an agreement to save Stormont still exists.
Theresa May spoke with DUP leader Arlene Foster and Sinn Fein leaders Mary Lou McDonald and Michelle O'Neill on Thursday night.
Mrs May has faced criticism for her intervention in the talks on Monday, with the DUP claiming her arrival at Stormont proved to be a "distraction" that stalled momentum in the process.
On Thursday, a No 10 spokeswoman said: "The Prime Minister spoke to the leaders of Sinn Fein and the DUP on the phone earlier this evening about the recent phase of political talks in Northern Ireland.
"In both phone calls the Prime Minister expressed her disappointment that an agreement had not yet been reached despite recent progress being made by everyone involved.
"She reiterated how the Government remains absolutely committed to restoring an executive and that it is still her firm belief that restoring devolved government - so that democratically elected politicians can make locally accountable decisions - is in the best interests of everyone in Northern Ireland.
"The Prime Minister was clear that she still believes the basis for an agreement exists and recognised the leadership both parties had shown over past weeks and months.
"She urged them both to reflect on the recent phase of talks in order to find the best way forward.
"On next steps, the Prime Minister was clear that the UK Government has a responsibility to consider what needed to be done to protect the interests of Northern Ireland in the ongoing absence of an executive.
"She added that the Northern Ireland Secretary would continue to work closely with the parties, and the Irish government in accordance with the three-stranded approach, to support that process."