Northern Ireland's First Minister Peter Robinson last night warned he would collapse the Stormont Assembly if Sinn Fein did not honour the landmark agreement on policing and parades, but stressed he was confident republicans would deliver.

The Democratic Unionist leader said he was sure his partners in the power-sharing government were fully committed to the deal and did not expect to need the nuclear option.

"I think everybody has recognised that there are methods available, particularly to the two main parties in this Assembly, which can be used at any and all times if they wish to bring down institutions to make sure that things don't work, to act as vetoes," he told the assembly as he outlined details of Friday's settlement on devolving law and order powers.

Elaborating on his cryptic reference last week to the "secret device" he had to ensure Sinn Fein compliance, Mr Robinson cited the example of former Ulster Unionist leader David Trimble, who triggered the suspension of the Assembly after resigning on the issue of IRA decommissioning, and recent rumours of a similar move by Sinn Fein Deputy First Minister Martin McGuinness.

"It's well recognised that just as the leader of the Ulster Unionist Party once wrote out his post-dated resignation and eventually caused a suspension of the Assembly, just as it was reported in the newspapers that the Deputy First Minister was prepared to resign - and unquestionably if he had done we would have probably ended up with something worse than suspension of the Assembly, probably the collapse of the Assembly - so too have I the ability to do exactly the same things as both of them if I felt if there was bad faith on the part the signatories to this agreement," he said.

But he added: "There has to be a time in any negotiating process when you look across the table at the person you are negotiating with and you make a determination whether you believe that that person is seriously going to keep the commitment that they are entering into and quite frankly if you don't believe that they are going to do so, you should walk away from the table and have nothing to do with it."

The DUP and Sinn Fein agreement hammered out at the marathon summit at Hillsborough Castle, Co Down, set out a number of interlocking processes around devolution and changes to parade management structures that are intended to culminate with the transfer of justice powers from Westminster on April 12.

As both leaders addressed the Assembly on the deal this afternoon, Mr McGuinness insisted his party would not shirk its responsibilities.

"It is much better to move forward in a positive vein than looking at the black side," he said.

"I travel with hope and optimism, I look on the bright side and whenever this party makes commitments, we are absolutely dedicated to keeping those commitments."

Mr Robinson made a passionate case for backing the agreement in the chamber of Parliament Buildings as the first steps in implementing it got under way.

A working group made up of three DUP and Sinn Fein members to find a way forward on the parading dispute held its first discussions, while Mr Robinson and Mr McGuinness held talks with other political leaders about potential candidates for the new Justice Minister. The nationalist SDLP is the only party to nominate for the post thus far.

Afterwards, the DUP leader referred to his recent personal crisis involving his wife Iris's infidelity as he briefed the Assembly on the deal.

"Over these last months I have had plenty of drama in my life," he said.

"I could have walked away very easily - every brain cell and every blood cell cried out to me to watch from the sidelines rather than the frontline. I'm here because of a sense of duty, because I believe that what we are doing actually matters, that what we are trying to achieve in this Assembly is something for the betterment for all the people in Northern Ireland... if other people in this Assembly don't collectively agree with that, then quite frankly we'd all be far better packing our bags and going home."

One of Sinn Fein's representatives on the new parading body, Gerry Kelly, said they would meet on a daily basis to formulate a framework within the group's tight three-week deadline.

"We have not said that we will sort out the issue of contentious parades in two or three weeks, we will try to put together a structure that will make it easier," he added.

"It has to be sorted out between the residents and the people who march in those areas, making dialogue and resolution easier.

"We are not trying to bring forward some miracle."

Nationalist residents in mainly working-class areas like north Belfast and Portadown, Co Armagh, oppose Orange Order processions past their houses because they see them as triumphalistic.

Members of the loyal orders accuse householders of going out of their way to be offended and maintain it is their traditional right to demonstrate on the streets.

The working group will seek to design a new system that will involve a greater emphasis on local accommodation. It will also attempt to create a better framework for mediation and a new system of adjudication.

The government-appointed Parades Commission has been criticised by unionists for barring Orange demonstrations like that at Drumcree, Portadown, from walking along the mainly nationalist Garvaghy Road.

The new Assembly group has been criticised by the SDLP for not including the other parties and non-political experts.

Mr Kelly, a junior minister in the Office of the First Minister and Deputy First Minister, said they planned to involve other "stakeholders" but were reporting to First Minister Robinson and Deputy First Minister McGuinness.

Lagan Valley DUP MLA Jeffrey Donaldson, who is also a member of the group, said: "The decisions we take as a working group will not be taken in a vacuum.

"They will be informed and influenced by those involved in parading, that is why we will be listening to those views and reflecting them in the final agreement." He added a resolution to the issue was essential to build public confidence.

"We will stick with the timetable agreed at Hillsborough, we want to get agreement in principle on how we move the parades issue forward," he added.

Elsewhere at Stormont, leaders from all five main parties in the Assembly met as part of a process of deciding who will be the first Justice Minister in 38 years.

Alliance Party head David Ford is the favourite but was not nominated today because he wants outstanding issues like developing better community relations dealt with.

He published proposals for a programme for government for policing and justice but said he would not allow himself to be put forward until his party's demands were met.

"We need to see progress on building a shared future for the people of Northern Ireland and we need to see a group conversation for the Department of Justice which would ensure that it delivers for all of the people of Northern Ireland," he said.

He admitted there had been some "frostiness" at today's Stormont meeting, but added that Sinn Fein and the DUP were prepared to engage.

"Since last Friday there has been constructive and positive engagement, certainly through yesterday. There remains work to be done but the Alliance Party remains committed to doing that," he added.

His party's proposals said there were considerable inefficiencies and cost pressures within the criminal justice system and work to manage scarce resources more efficiently and effectively must be intensified to ensure a sustainable level of public service.

The SDLP nominated its North Belfast Assembly member, Alban Maginness, a former lawyer, for the post.

Leader Margaret Ritchie said: "There should not be any departure from the democratic principles as enshrined in the Good Friday Agreement." She also said she had agreed to participate in more bilateral meetings with Mr Robinson and Mr McGuinness.

Ms Ritchie is also holding talks with Irish Foreign Affairs Minister Micheal Martin in Dublin tomorrow.

Ulster Unionist leader Sir Reg Empey said little substantive progress had been made today but that was not expected at the first meeting.