The British and Irish prime ministers, David Cameron and Enda Kenny, have arrived in Belfast to oversee multi-party negotiations on saving Northern Ireland's power-sharing government, a peacemaking alliance at risk of collapse.

Cameron says their intervention in the 9-week-old negotiations signals that the effort has "reached a crucial phase." The British leader says both premiers will push to persuade rival parties to sustain a coalition uniting the Protestant majority and Catholic minority, the central achievement of the 1998 Good Friday accord.

Over the past year the two principal parties, the Irish Catholics of Sinn Fein and British Protestants of the Democratic Unionist Party, have taken opposite sides in a growing list of political, financial and cultural disputes. They have made Dec. 24 an informal deadline for a deal or defeat.