As world leaders began arriving for the start of the G7 summit, there was already talk that the group would be better described as the G6+1.

At the heart of the rift, US President Trump's new tariffs on steel and aluminium imports - and the threat of a global trade war.

French President Emmanuel Macron called on the G7 to stand up to the Trump administration's stance.

"So maybe the American president doesn't care about being isolated today but neither do we mind signing a six-country agreement if need be. Because these six represent values, they represent an economic market which has the weight of history behind it and which is now a true international force," he tweeted.

President Trump soon responded on Twitter - accusing Canada and the EU of keeping the US out of their markets.

"Please tell Prime Minister Trudeau and President Macron that they are charging the US massive tariffs and create non-monetary barriers. The EU trade surplus with the US is $151 billion and Canada keeps our farmers and others out. Look forward to seeing them tomorrow."

Trump followed up by accusing Canada and the EU of imposing "massive" tariffs which were "totally unfair" to the American economy.

Twice he attacked the Canadian prime minister on Twitter on Thursday and officials say the mood is likely to be exceptionally tense.

Trudeau had wanted the summit to help find common solutions to issues such as growth and environmental protection, and officials say he remains optimistic.

The US president says the tariffs on aluminium and steel are necessary to protect American industry. Canada and the European Union have denounced them as illegal and are preparing retaliatory measures.

British Prime Minister Theresa May took a more measured tone than Macron, telling reporters she wanted the EU to use restraint in retaliation to the US tariffs and saying that the response must be proportionate and legal.

Security is tight at the Quebec resort where the summit takes place.

Protesters are expected to stay away and instead hold events in Quebec City.

Donald Trump is due to leave the summit on Saturday, before it formally ends, to fly to Singapore for a meeting with North Korean leader Kim Jong Un.

G7 leaders have praised the US president for his efforts to stabilise the Korean peninsula, but are unhappy that he pulled out of the Iran nuclear deal.