Sinn Féin leader Mary Lou McDonald said she's "absolutely convinced" a deal can be reached with the DUP, to restore power-sharing in the North.
Talks to get the Stormont assembly back up and running began last week.
While Ms McDonald and British Prime Minister Theresa May are to speak by phone tomorrow.
Ms McDonald said: "The two governments are not spectators or referees in this process, they are co-equal guarantors of the people's agreements and rights.
"The actions of the British government, their deal with the DUP, imposition of Brexit and refusal to honour their obligations on the legacy issues of the past, has dogged this process for the last two years.
"I will be raising these issues directly with the British Prime Minister on Monday".
But she's upbeat on the prospect of finding a solution.
"I am absolutely convinced that these issues can be resolved - I'm convinced of that because we managed to land on an accommodation before.
"Unfortunately at that time, we didn't have a partner in unionism to deliver that deal: I very, very much hope that this time round we can not alone strike a deal, but that we can find partners in unionism".