Under Secretary of Defense for Policy Elbridge Colby arrived in South Korea on Monday and is seen as a key proponent of President Donald Trump's "America First" foreign policy.

That policy -- detailed in Washington's 2026 National Defense Strategy (NDS) released last week -- calls for the United States to prioritise deterring China and for long-standing US allies to take "primary responsibility" for their own defence.

Arriving in Seoul on his first overseas trip as the Pentagon's number three official, Colby in a post on X called South Korea a "model ally".

And he praised President Lee Jae Myung's pledge to spend 3.5 percent of the country's GDP on the military.

That decision, he told a forum, "reflects a clear-eyed and sage understanding of how to address the security environment that we all face and how to put our storied and historic alliance on sound footing for the long haul," according to South Korea's Yonhap News Agency.

"Such adaptation, such clear-eyed realism about the situation that we face and the need for greater balance in the sharing of burdens, will ensure that deterrence remains credible, sustainable and resilient in this changing world," he added, according to the agency.

Colby also met Monday with South Korea's defence and foreign ministers, who touted Seoul's development of nuclear-powered attack submarines as proof the country was taking more responsibility for its defence.

Details remain murky on where the nuclear submarines will be built, however.

South Korea's leader said last month it would be "extremely difficult" for them to be built outside the country.

But Trump has insisted they will be built in the United States.

Longstanding treaty allies, ties between the United States and South Korea were forged in the bloodshed of the Korean War.

Washington still stations 28,500 troops in South Korea as a deterrent against the nuclear-armed North.