U.S. President Donald Trump has put a price tag on Canada joining his proposed Golden Dome missile defence system — and renewed his annexation threat in the process.

Trump posted on his Truth Social platform on Tuesday that it will cost Canada $61 billion US to join the Golden Dome "if they remain a separate, but unequal, Nation," but will cost nothing "if they become our cherished 51st State."

He claimed Canada is "considering" the offer.

This comes on the same day as the throne speech was delivered in Parliament by King Charles, during which he asserted Canada's sovereignty and Prime Minister Mark Carney committed his government to joining a major European defence rearmament plan.

Carney told CBC's Power & Politics in an interview following the speech that he hopes Canada will be able to join ReArm Europe by July 1, in a step to reduce the country's dependence on the United States for weapons and munitions. 

WATCH | Mark Carney talks about European military buildup:

Ottawa in talks on European military buildup

Prime Minister Mark Carney said during a one-on-one interview with Power & Politics host David Cochrane on Tuesday that he’s been having ‘conversations of increasing specificity’ with major European partners about Canada being part of a build-out of their defence industrial base. Carney said he wants to see ‘something concrete there’ by Canada Day.

Trump's social media post also marks the first time he has levelled his "51st state" threat since meeting with Carney in the Oval Office in Washington, D.C., on May 6.

It could reignite tensions between the two nations, coming days after Pete Hoekstra, the U.S. ambassador to Canada, said that the annexation saga was over.

"Move on. If the Canadians want to keep talking about it — that's their business," Hoekstra told CBC News last week. "I'm not talking about it; Donald Trump is not talking about it."

In response to Trump's post, a spokesperson with the Prime Minister's Office said federal government officials are discussing security-related issues with the U.S., including the Golden Dome and the North American Aerospace Defence Command (NORAD).

"The prime minister has been clear at every opportunity, including in his conversations with President Trump, that Canada is an independent, sovereign nation, and it will remain one," the spokesperson said in a statement to CBC News.

"Canadians gave the prime minister a strong mandate to negotiate a comprehensive new security and economic relationship with the United States. To that end, the prime minister and his ministers are having wide-ranging and constructive discussions with their American counterparts. These discussions naturally include strengthening NORAD and related initiatives such as the Golden Dome."

Dome could cost hundreds of billions

The Golden Dome plan is modelled after the Iron Dome that protects Israel from rockets, but it aims to be more complex and hundreds of times bigger.

Trump and other U.S. officials say the system would be able to block missiles fired from other countries and from space, weaving together existing technologies with future tech that does not yet exist. He said last week that Canada wants to "hook in" to the system and would pay its "fair share."

The president has said the dome will cost $175 billion US and be built in three years, but some experts say it would likely take closer to 20 years — requiring buy-in from multiple administrations — and cost as much as $1 trillion.

The plan has been condemned by China and raised concerns about the possibility of igniting a space-based arms race with China, Russia and other countries that have or are developing military space capabilities.