"We have agreed the most ambitious submarine program in history, AUKUS," the King told US politicians.
"And we do so in partnership with Australia, a country of which I am also immensely proud to serve as sovereign.
"We do not embark on these remarkable endeavours together out of sentiment.
"We do so because they build greater shared resilience for the future, so making our citizens safer for generations to come."
Under the 2021 security deal, Australia will acquire nuclear-powered submarines with support from the UK and the US, alongside co-operation on military technology.
Australia has been promised at least three Virginia-class submarines from the US in the early 2030s, before a new fleet of vessels is built for delivery from the 2040s.
Under AUKUS, Australia eventually plans to build five submarines in South Australia, using US technology and parts made in Britain.
The US last year concluded a review of the AUKUS agreement aimed at ensuring "its long-term success" and "alignment" with President Donald Trump's 'America First' agenda.
with AAP
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