US President Donald Trump says that if Canada becomes the 51st US state, it can join the Golden Dome missile defense system for free.

Trump wrote on social media on Tuesday, "I told Canada, which very much wants to be part of our fabulous Golden Dome System, that it will cost $61 Billion Dollars if they remain a separate, but unequal, Nation, but will cost ZERO DOLLARS if they become our cherished 51st State."

Trump has already said he aims for the system to become operational by January 2029, before the end of his term. The next-generation missile defense shield would be deployed not only on land and at sea, but also in space. The US administration says building the system will cost 175 billion dollars.

It is apparently designed to intercept attacks from countries including China and Russia, which are pushing for missile development.

Canadian Prime Minister Mark Carney has indicated his government will consider making a financial contribution to the project. But he has firmly rejected Trump's suggestion that Canada become a US state.