By Staff, Agencies Northern Ireland's power-sharing government put aside factional differences on Thursday to call for calm after frustration among pro-British unionists over post-Brexit trade barriers helped trigger some of the worst violence in the region in years. Despite the appeals, clashes spread further into Irish nationalist areas on Thursday night where police responded to petrol bomb and stone attacks with water cannon.

The White House joined the British and Irish governments in urging calm.

Hundreds of youths in the British province's capital Belfast set a hijacked bus on fire and attacked police with stones on Wednesday in scenes reviving memories of decades of sectarian and political strife that claimed some 3,600 lives prior to a 1998 peace deal.

A week of violence has injured 55 police officers and seen boys as young as 13 and 14 arrested on rioting charges.

"We are gravely concerned by the scenes we have all witnessed on our streets," the compulsory coalition, led by rival pro-Irish Catholic nationalists and pro-British Protestant unionists, said in a statement.

"While our political positions are very different on many issues, we are all united in our support for law and order and we collectively state our support for policing," the statement said