Trump tweeted: "Based on Justin's false statements at his news conference, and the fact that Canada is charging massive Tariffs to our U.S. farmers, workers and companies, I have instructed our U.S. Reps not to endorse the Communique as we look at Tariffs on automobiles flooding the U.S. Market!"
Larry Kudlow, director of the National Economic Council, lashed out at Canadian Prime Minster Justin Trudeau as "amateurish" and "sophomoric" for a news conference he gave after Trump left the Group of Seven summit on Saturday.
In the same interview, Merkel said the European Union was preparing to implement counter-measures against US tariffs on imported steel and aluminum. Trudeau, who threatened retaliatory tariffs, also said Canada would not be "pushed around".
"We're the piggy bank that everybody is robbing", Trump said.
In response, Trump tweeted on his way to Singapore that he instructed USA representatives not to endorse the communique. "The Prime Minister said nothing he hasn't said before - both in public, and in private conversations with the president".
Kudlow also accused Trudeau of trying to undermine the USA president at a critical time before his summit with the reclusive Kim on Tuesday. This has raised concerns at home and overseas, and shortly after Mr Trump tweeted, respected Republican Senator John McCain responded.
"The reality is that even under the best of circumstances, Canada is a middle power and, you know, when you're a middle power, you have to get along with larger powers".
Merkel, whose office over the weekend released a photo of leaders and aides which illustrates the divide between Trump and his allies, said she had "tried hard to find a compromise and we fought hard for it. this was an important announcement".
Trump's decision to withdraw his support for the communiqué has drawn sharp criticism from Germany on Sunday.
Copies that begin "We, the Leaders of the G-7." were distributed in the press room stamped "Approved".
The remarks have enraged U.S. President Donald Trump.
The Trudeau government has announced it will impose retaliatory tariffs on metals and a range of other USA products by July 1.
Mr Trump's state visit in November will coincide with the 6th ASEAN-US Summit and 13th East Asia Summit. But before he left a text had already been agreed that emphasised that on climate change the G7 has become a G6, with separate paragraphs setting out the differing positions of the U.S. and Canada, France, Germany, Italy, Japan, the United Kingdom and the EU. The talks have floundered over several issues, including Trump's insistence on a clause that would end NAFTA every five years unless all three countries agree to sustain it.
Ontario Conservative MP John Brassard says he's heard over the weekend from anxious businesses that use aluminum and steel, adding that there is real worry that there will be serious job implications within a couple of weeks.
"Canada does not believe that ad hominem attacks are a particularly appropriate or useful way to conduct our relations with other countries", said Canadian foreign affairs minister Chrystia Freeland, according to the Toronto Star.
Trump also pitched for a "reciprocal tax" on countries that he said "abuse" their trade relationships with the US.
"We don't want to stand alone in the world", Feinstein said on CNN's State of the Union.