US President Donald Trump has accused Canada of wanting “the benefits of being a [US] State, without being one” after trade talks between the two countries collapsed.

Canadian Prime Minister Mark Carney said at the weekend his nation is in a trade war with the US, announcing tariffs on American goods in retaliation for new 50 per cent US tariffs.

“You’re at war when you get attacked; We got attacked,” Carney said, in response to a journalist who asked why it felt like a trade war.

“We cannot accept what they’ve offered, and we will not give what they’ve asked.”

The new Canadian tariffs would come into force on September 8, he said.

The US meanwhile is looking ahead at how to respond further to the retaliation.

Trump’s latest comments echo his repeated references to making Canada the “51st state” of America since returning to office, sparking frustration among its leaders and residents and raising tensions between the two neighbours.

“They have also charged our great farmers, for many years, massive amounts of Tariffs,” Trump wrote on his social media platform Truth Social.

Carney said the planned Canadian levies focused on sectors such as steel, dairy products, agricultural equipment and electrical appliances.

“This is a focused response – a focused response to protect and defend our industries and allow them to compete with US products in the Canadian market,” he said.

Carney’s move follows new 50 per cent US tariffs on a range of Canadian goods worth around $US20 billion ($A28 billion), including wine, ice hockey sticks, furniture and dairy products. They came after a deadline set by Trump for a deal to be reached passed without an agreement.

“This is a missed opportunity for Canada to partner with the United States,” US Trade Representative Jamieson Greer told Fox News as Canada announced the retaliation.

“We don’t have new talks planned with the Canadians. We’re moving forward with measures that respond to Canadian retaliation.”

On Sunday (local time), US Transportation Secretary Sean Duffy said Carney would “come to the table” on trade because the US tariffs would be “devastating”.

“To think that they’re going to go to war with Donald Trump and actually win that war with the US, I think it’s foolish on their part,” Duffy told “ox News Sunday.

Negotiators from Canada and the US had discussed an agreement late into Friday to try to avert Trump’s tariffs.

There was talk of reviving the Keystone Pipeline extension, a project long championed by Trump to transport Canadian oil to the US but dropped on objections by environmental groups and Native American tribes.

In return, Canada’s provinces could lift their ban on US alcohol sales.

But after the talks broke down, the two sides accused each other of having prevented an agreement through last-minute demands.

Carney said the US was constantly finding new reasons to justify its tariffs on Canada, citing fentanyl, smoke from forest fires, taxes on US tech companies, and a Canadian advert that quoted former US president Ronald Reagan.

Once the US tariffs kicked in, Carney vowed to respond “dollar for dollar”.

Earlier, he said he had decided to suspend trade negotiations with the US and directed Canada’s negotiators to return to Ottawa, in a statement published shortly before the deadline expired.

“They have worked hard, in good faith, to defend the interests of Canadians throughout these negotiations up until the very last minute,” Carney said.

“However, last-minute changes in the US proposed terms were unfair, uneconomic, and called into question the reliability of any deal.”

The tariffs are the latest milestone in Trump’s remaking of US trade policy since he returned to office last year.

-with AAP