Quebec City: Bashing the leader of one of America's venerable allies, the White House escalated its trade tirade and levelled more withering and unprecedented criticism on Sunday against Canada's Prime Minister, branding Justin Trudeau a back-stabber unworthy of President Donald Trump's time.

"There's a special place in hell for any foreign leader that engages in bad faith diplomacy with President Donald J. Trump and then tries to stab him in the back on the way out the door," Trump trade adviser Peter Navarro said in an interview broadcast nationally in the US.

The verbal volleys by Navarro and Trump's top economic adviser, Larry Kudlow, picked up where Trump left off on Saturday evening with a series of tweets from Air Force One en route to Singapore for his nuclear summit on Tuesday with North Korea's Kim Jong-un.

Just as the Trudeau-hosted G7 meeting of the world's leading industrialised nations had seemed to weather Trump's threats of a trade war, the US President backed out of the group's joint statement that Trudeau said all the leaders had come together to sign.