BELFAST, Northern Ireland — Northern Ireland’s political parties began talks Monday to try to save their power-sharing government after revelations about the involvement of Irish nationalist militants in a murder prompted the pro-British first minister to step aside.

That move effectively froze the power-sharing structures created under a 1998 peace deal. Irish nationalist party Sinn Fein has said a full collapse would lead to a “very real prospect” of increased violence.

The head of the Democratic Unionist Party Peter Robinson and Sinn Fein’s Martin McGuinness were joined for the talks — expected to last several weeks — by representatives of the British and Irish governments.

The 1998 peace deal largely ended three decades of sectarian violence between Catholics who want a united Ireland and Protestants who want Northern Ireland to remain British. More than 3,600 people died in the conflict.

But one of the key planks of that deal was undermined last month when police said the Irish Republican Army was likely involved in the murder of a former operative, despite assurances from Sinn Fein that the group had “left the stage.”