Talks at the Group of Seven summit will not be easy as the United States is in conflict with the rest of the group on trade, climate change and the nuclear deal with Iran, the chairman of European Union leaders Donald Tusk said on Friday.

"Our discussions here is Charlevoix will be far from easy," Tusk told a news conference before the start of the two-day talks of leaders from the United States, Japan, Canada, France, Germany, Italy and Britain.

"It is evident that the American president and the rest of the group continue to disagree on trade, climate change and the Iran nuclear deal. What worries me most is that the rules-based international order is being challenged," he said.

"Quite surprisingly not by the usual suspects, but by its main architect and guarantor - the United States. Naturally we cannot force the US to change its mind," Tusk said.

US President Donald Trump lashed out at Canada and the European Union on Friday, setting the tone for a hostile summit and raising the spectre of a trade war that has unnerved Washington's top allies as well as investors.

The confrontation over US tariffs on steel and aluminium imports threatened to rupture the G7, which during its 42-year history has traditionally sought to find consensus on the economy and other issues.

Trump, who aides said has scant interest in multilateralism, resumed his tirade against G7 members and their "unfair trade practices" early on Friday. The White House said he would leave the two-day summit in Canada four hours earlier than originally planned.

The US leader will miss talks about climate change and clean energy, and will have left Canada by the time Canadian Prime Minister Justin Trudeau and other leaders begin closing news conferences likely to be laden with criticism of Washington's trade policy.

"We're going to deal with the unfair trade practices. If you look at what Canada, and Mexico, the European Union - all of them - have been doing to us for many, many decades. We have to change it. And they understand it's going to happen," Trump said before departing for the summit in La Malbaie, Quebec.

"We are going to do very well. Now, if we're unable to make a deal we'll terminate NAFTA, we'll have a better deal," Trump said, adding that the United States would be better off without trade pacts such as the North American Free Trade Agreement.

Trump also said that Russia should be allowed again to attend G7 summits.

A spokesman for Canada's Trudeau said that Ottawa's position against such a move had not changed.

A senior British government source also reacted cooly to the proposal: "Russia needs to change its approach before any conversation about it rejoining the G7 can begin. The (prime minister) has always said we should engage with Russia but beware," the source told Reuters.

"We should remind ourselves why the G8 became the G7 - it was after Russia illegally annexed Crimea.... Since then we have seen malign activity from Russia in a whole variety of ways, including on the streets of Salisbury in the UK."

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