US President Donald Trump dramatically withdrew his endorsement of a joint statement at the end of a G7 summit in Canada today in a row over trade, accusing the summit's chairman Justin Trudeau of dishonesty.

"Based on Justin's false statements at his news conference, and the fact that Canada is charging massive Tariffs to our US farmers, workers and companies, I have instructed our US Reps not to endorse the Communique," Trump said in Tweet sent from on board Air Force One.

Trump, who is on his way to Singapore for a summit with North Korea's leader, also reiterated warnings that his administration was mulling tariffs "on automobiles flooding the US market" in a move that would be primarily aimed at another G7 member, Germany.

The US president then had angry words for Trudeau, saying the Canadian prime minister had "acted so meek and mild" in their joint discussions only to later pledge in a press conference that he would not be pushed around.

"Very dishonest & weak," Trump said, adding that recently imposed US tariffs on imports of Canadian steel and aluminium were in response to similar measures imposed on American dairy products by Canada.