The leader of the Democratic Unionist Party (DUP), Peter Robinson, says there will be no meetings of the Northern Ireland Executive unless in “exceptional circumstances”, as the crisis Stormont was plunged into after the killing of a former IRA member, escalates.

Mr Robinson, who is also First Minister of the assembly, said the DUP's ministers would focus on talks to try to resolve the crisis that are due to begin on Tuesday.

Police have said they believe members of the Irish Republican Army were involved in the murder of Kevin McGuigan Snr, however added there was no evidence at this stage to suggest the shooting was sanctioned by the organisation.

Republican party Sinn Féin rejected the police’s accusation and said the IRA had "gone" and was not "coming back".

However, Mr Robinson said the DUP was not "prepared to continue as if nothing had happened", the BBC reported.

"A man has been murdered at the hands of those linked to a party of government. This is unacceptable," Mr Robinson said.

Traditional Unionist Voice (TUV) leader Jim Allister criticised the DUP's actions.

"An executive that doesn't work, that doesn't deliver, isn't going to meet - big deal, that's it, that's the big DUP response to murder," he said.

"This is an executive that once didn't meet for six months and nobody noticed."

According to a Downing Street spokesperson, Prime Minister David Cameron and Taoiseach Enda Kenny had spoken on Wednesday about the situation in the north of Ireland.

"They agreed that the current situation in Northern Ireland is serious and without urgent progress there is a real threat to the stability of the devolved institutions," the spokesperson said.