The G7 summit ended in farce and a renewed threat of global trade war as US President Donald Trump abruptly rejected the text of a consensus statement and bitterly insulted the Canadian host.

Just minutes after a joint communique that had been approved by the leaders of the Group of Seven allies on Saturday was published in Canada's summit host city Quebec, Trump launched a Twitter broadside from aboard Air Force One.

The US leader left the meeting early en route for Singapore and a historic nuclear summit with North Korea's Kim Jong Un, only to take exception to comments made by Canada's Prime Minister Justin Trudeau at a news conference.

“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 as we look at Tariffs on automobiles flooding the US Market!” Trump tweeted.

“PM Justin Trudeau of Canada acted so meek and mild during our @G7 meetings only to give a news conference after I left saying that ... he 'will not be pushed around.' Very dishonest & weak.”

It was an unprecedented attack on the leader of the US neighbour and ally.

'Insulting'

Earlier, Trudeau had told reporters that Trump's decision to invoke national security to justify US tariffs on steel and aluminum imports was “kind of insulting” to Canadian veterans who had stood by their US allies in conflicts dating back to World War I.

“Canadians are polite and reasonable but we will also not be pushed around,” he said.

Trudeau said he had told Trump “it would be with regret but it would be with absolute clarity and firmness that we move forward with retaliatory measures on July 1, applying equivalent tariffs to the ones that the Americans have unjustly applied to us,”

After Trump's angry tweets, Trudeau's office issued a brief response: “We are focused on everything we accomplished here at the G7 summit. The Prime Minister said nothing he hasn't said before — both in public, and in private conversations with the President.”

The outburst against Trudeau, and by association the other G7 members, is only the latest incident in which Trump has clashed with America's closest allies, even as he has had warm words for autocrats like Kim and Russia's Vladimir Putin.

Also read: EU ripostes as Trump kicks off transatlantic trade war

French President Emmanuel Macron's office reacted Sunday by saying that “international cooperation cannot be dictated by fits of anger and throwaway remarks”.

Reneging on the commitments agreed in the communique showed “incoherence and inconsistency”, it said in a statement.

Shortly after Trump tweeted, respected Republican Senator John McCain responded.

“To our allies: bipartisan majorities of Americans remain pro-free trade, pro-globalization & supportive of alliances based on 70 years of shared values.

"Americans stand with you, even if our president doesn't," he tweeted.

'Creative babbling'

Russian President Vladimir Putin, in China on Sunday for a summit with his Chinese counterpart Xi Jinping, dismissed G7 calls for Moscow to stop what the group described as attempts to undermine democracy and support for the Syrian regime.

“I believe it's necessary to stop this creative babbling and shift to concrete issues related to real cooperation,” Putin told reporters.

The G7 also endorsed Britain's accusation that Moscow was behind the poisoning attack in England on former double agent Sergei Skripal and his daughter - but Putin said they had “again” failed to provide evidence that Russia was behind the attack.

Russia was kicked out of the group in response to its 2014 annexation of Crimea. Trump earlier said that the club would be better off if it brought Russia back.

World's 'piggy bank'

When Trump left Quebec it was thought that a compromise had been reached, despite the tension and determination of European leaders Macron and Chancellor Angela Merkel of Germany to push back on Trump's assault on the world trade system.

Officials from European delegations quickly leaked copies of the joint statement, and it was published online moments before Trump tweeted.

On board Air Force One an AFP reporter was told that Trump had approved the agreement, only to be told later of the tweets. A senior US administration official said that Trump had been angered by Trudeau's comments.

The outburst suggested that any deal had collapsed and his threat to impose sanctions on car imports will outrage his ostensible allies — especially Germany and Canada, major exporters to the US market.

Trump claimed that tariffs were necessary because the US has been exploited as the world's “piggy bank” under existing arrangements, but his counterparts were determined to protect “rules-based” international trade.

'The gig is up'

The joint communique that was thrashed out over two days of negotiations vowed that members would reform multilateral oversight through the World Trade Organization (WTO) and seek to cut tariffs.

“We commit to modernise the WTO to make it more fair as soon as possible. We strive to reduce tariff barriers, non-tariff barriers and subsidies,” it said, reflecting the typical language of decades of G7 statements.

A German government spokesman said that his country “stands behind the communique which was collectively agreed upon”. But Trump had already said he would not hesitate to shut countries out of the US market if they retaliate against his tariffs.

“The European Union is brutal to the United States... They know it,” he insisted in his departing news conference. “When I'm telling them, they're smiling at me. You know, it's like the gig is up.”

European officials said Trump had tried to water down the language in the draft communique on the WTO and rules-based trade. In the end, that language stayed in and it was only on climate change that no consensus was reached.