US president Donald Trump took more swipes at Canada and its prime minister over trade issues as he settled in for a multi-day summit with North Korea in Singapore, contending that “fair trade is now to be called fool trade if it is not reciprocal”.

Mr Trump roiled the G7 meeting in Canada by first agreeing to a group statement on trade, only to withdraw from it while complaining that he had been blindsided by Canadian Prime Minister Justin Trudeau’s criticism of his tariff threats at a summit-ending news conference.

As he flew from Canada to Singapore on Saturday night, Mr Trump displayed his ire via Twitter, which he also employed to insult Mr Trudeau as “dishonest” and “weak”.

The attack on a long-time ally and its leader drew sharp criticism.

German Chancellor Angela Merkel, who also attended the summit, told German public television that she found Mr Trump’s tweet disavowing the G7 statement “sobering” and “a little depressing”.

She also said the European Union would “act” against the US trade measures.

Unbowed, Mr Trump tweeted anew on Monday morning from Singapore: “Fair Trade is now to be called Fool Trade if it is not Reciprocal. According to a Canada release, they make almost 100 Billion Dollars in Trade with U.S. (guess they were bragging and got caught!). Minimum is 17B. Tax Dairy from us at 270%. Then Justin acts hurt when called out!”

He added: “Why should I, as President of the United States, allow countries to continue to make Massive Trade Surpluses, as they have for decades, while our Farmers, Workers & Taxpayers have such a big and unfair price to pay? Not fair to the PEOPLE of America! $800 Billion Trade Deficit…And add to that the fact that the U.S. pays close to the entire cost of NATO-protecting many of these same countries that rip us off on Trade (they pay only a fraction of the cost-and laugh!). The European Union had a $151 Billion Surplus-should pay much more for Military!”

And he brought in Mrs Merkel’s government, saying: “….Germany pays 1% (slowly) of GDP towards NATO, while we pay 4% of a MUCH larger GDP. Does anybody believe that makes sense? We protect Europe (which is good) at great financial loss, and then get unfairly clobbered on Trade. Change is coming!”

Earlier, the White House escalated the initial tirade and levelled more withering and unprecedented criticism against Mr Trudeau, branding him a back-stabber unworthy of Mr 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,” Mr Trump trade adviser Peter Navarro said in an interview nationally broadcast in the United States.

Canada’s foreign minister, Chrystia Freeland, said her country “does not conduct its diplomacy through ad hominem attacks”.

The verbal volleys by Navarro and Trump’s top economic adviser, Larry Kudlow, picked up where Trump had left off on Saturday evening. Mr Kudlow suggested Mr Trump saw Mr Trudeau as trying to weaken his hand before that meeting, saying the president would not “let a Canadian prime minister push him around. … Kim must not see American weakness”.

Mr Trudeau, who had said at the news conference that Canada would retaliate over new US tariffs, did not respond to questions about Mr Trump when the prime minister arrived at a Quebec City hotel on Sunday for meetings with other world leaders.

President Donald Trump meets with Canadian Prime Minister Justin Trudeau during the G-7 summitPresident Donald Trump meets with Canadian Prime Minister Justin Trudeau during the G-7 summit (AP Photo/Evan Vucci)

Spokesman Cameron Ahmad said on Saturday night that Mr Trudeau “said nothing he hasn’t said before — both in public and in private conversations” with Mr Trump.

And Roland Paris, a former foreign policy adviser to Trudeau, jabbed at Mr Trump on Twitter: “Big tough guy once he’s back on his airplane. Can’t do it in person. … He’s a pathetic little man-child.”