G7 leaders will pressure US President Donald Trump at a summit this week on tariffs he has imposed on metals imports but must remain civil as they try to persuade him to change his mind, French President Emmanuel Macron has said.

Mr Macron's comments reflect tensions over trade that threaten to overshadow the two-day summit in Quebec tomorrow and on Saturday.

Non-US G7 members say they will be firm as they confront Mr Trump but do not want a rupture that could cause a major trade war.

"In this environment, above all we always have to stay polite, stay productive and try to convince (them), to keep the United States on board because they are our historical ally and we need them," said Mr Macron.

Canadian Prime Minister Justin Trudeau said the leaders would air their unhappiness in what he called a polite cordial context and expressed optimism about the summit.

Earlier, the White House's top economic adviser said Mr Trump would not be backing down from the tough line he has taken on trade, setting the stage for a showdown at the summit.

The meeting tomorrow and on Saturday in Charlevoix, Quebec, will be the first chance G7 leaders have had to confront Mr Trump in person since US tariffs on steel and aluminum imports from Canada, Mexico and the European Union were imposed last week.

Donald Trump

That move unleashed fury in the Group of Seven industrialised nations and prompted quick retaliation from Canada and Mexico and a promise from the EU to do so as well, unnerving investors who fear a trade war that could derail the global economy.

The G7 groups Canada, the United States, Japan, Britain, Italy, France and Germany. The EU also attends.

Mr Trudeau, the summit host, and British Prime Minister Theresa May, who will also attend, are among those to sharply criticise the US tariffs as unjustified and punitive.

"There are disagreements. He's sticking to his guns. And he's going to talk to them," Larry Kudlow, Mr Trump's top economic adviser, told reporters in Washington.

Mr Trump, who has vowed to protect US industry and workers from what he describes as unfair international competition as part of an "America First" agenda, is due to hold bilateral meetings with Mr Trudeau and French President Emmanuel Macron during the summit, Mr Kudlow said.

A French presidency official said that while G7 members would raise their unhappiness over the tariffs with Mr Trump, they would not deliver an ultimatum that he drop them because the summit "isn't the place where you negotiate things like that".

The official, speaking to reporters after Mr Macron met Mr Trudeau for talks in Ottawa yesterday, said the six non-US G7 nations were united on the main topics to be discussed.

Mr Trump's meeting with Mr Trudeau could be particularly frosty, given the president's recent sharp criticism of Canadian trade policies and the anger in Ottawa over Washington's decision to justify its new tariffs on national security grounds.

"I know we're going to have some very, very frank conversations quite clearly around the table," Mr Trudeau told Global TV, adding he would convey Canada's displeasure over the tariffs personally when he meets Mr Trump in Quebec.

Two Canadian-based sources said divisions between the United States and other G7 members were so great that senior officials in charge of each nation's preparations planned to hold an unusual additional meeting tonight in a bid to find consensus.

G7 summits usually end with the host nation issuing a final communique, but Canada may decide not to do so, preferring instead to release a statement by Mr Trudeau summarising the talks, said the sources.