Sinn Fein accuses David Cameron over Good Friday deal
Sinn Fein president Gerry Adams urged David Cameron to fulfil his obligations under the Good Friday Agreement.
The British Prime Minister met a Sinn Fein delegation for the first time, with Mr Adams joined by Northern Ireland's Deputy First Minister Martin McGuinness.
Mr Adams said that Mr Cameron was aware there were "difficulties" in the process and would meet Sinn Fein again in the autumn.
Speaking outside the Palace of Westminster, Mr Adams said: "We made the point to him that he is not fulfiling his obligations in terms of the Good Friday Agreement, that the process is in a state of stagnation."
The meeting, and fresh talks at Stormont, come six months after marathon negotiations chaired by former US diplomat Richard Haass ended without agreement.
While draft proposals outlined by Dr Haass remain on the table, efforts to strike a deal in his absence have made little progress.
Mr Adams, who also met Labour leader Ed Miliband, warned that the height of the marching season in July could be a "potential disaster".
He criticised the "negative interventions" by Northern Ireland Secretary Theresa Villiers and said "what we need is this government to be a champion of the Good Friday Agreement".
Mr Cameron has insisted Northern Ireland's economic recovery is on course, a year into a government-backed stimulus package aimed at cementing political gains made in the post-conflict era.
A mainstay of the package was giving the Northern Ireland Executive the ability to borrow an additional £100m from the Treasury.
But discussions about the economy were overshadowed by a row about Sinn Fein blocking welfare reforms in Northern Ireland, which has led to financial penalties from the UK government.
First Minister Peter Robinson said he was "outraged" about the situation and claimed that the position adopted by Sinn Fein and Mr McGuinness would damage vital services.
Sinn Fein "doesn't get it or doesn't want to get it", he said.