U.S. President Donald Trump fired off a volley of tweets on Monday venting anger on NATO allies, the European Union and Canadian Prime Minister Justin Trudeau in the wake of a divisive G7 meeting over the weekend. The escalating clash over trade between Washington and some of its closest global partners cast a cloud over Trump's efforts to make history in nuclear talks in Singapore on Tuesday with Kim Jong Un of North Korea, one of America's bitterest foes.

Having left the Group of Seven summit in Canada early, Trump's announcement that he was backing out of the joint communique torpedoed what appeared to be a fragile consensus on the trade dispute between Washington and its top allies.

"Fair trade is now to be called fool trade if it is not reciprocal," said Trump, who flew from Canada to Singapore on Sunday to prepare for the first-ever summit between a U.S. and North Korean leader.