Blinken visited just as investigators were trying to arrest conservative President Yoon Suk Yeol, who has entrenched himself in his residence after being impeached for a failed attempt to impose martial law.

In a reminder of common challenges that go beyond domestic politics, North Korea on Monday fired a ballistic missile into the sea just as Blinken was holding meetings in Seoul, according to the South Korean military.

"Our military detected one projectile presumed to be a intermediate-range ballistic missile" launched toward the East Sea, South Korea's military said, referring to the body of water also known as the Sea of Japan.

After the missile flew around 1,100 kilometres (680 miles), the military said Seoul "strengthened surveillance and vigilance" for any more launches.

The missile appeared to have fallen into the water, according to Japan.

Seoul was "in close coordination with the US and Japan" about the launch, the South's military added.

The test comes two weeks before the inauguration of President-elect Donald Trump, who in his last term sought to woo North Korea with a unique personal diplomacy.

- Blinken's final tour -

Blinken, on the first stop of what will likely be his final trip as the top US diplomat, met acting president Choi Sang-mok, a technocrat in office just over a week, as well as Foreign Minister Cho Tae-yul, who is not under threat of impeachment.

With noisy protests gripping Seoul and audible from Blinken's hotel, he steered clear of wading into partisan politics.

Blinken reiterated the "ironclad commitment" of Washington to defending South Korea and spoke with Choi about "how both sides will work together to further strengthen bilateral cooperation and trilateral cooperation with Japan", a State Department statement said.

Until briefly imposing martial law on December 3, Yoon had been a darling of President Joe Biden's administration for his pro-US policies on the global stage.

Yoon delighted the United States by seeking to turn the page on decades of friction with Japan, a fellow US ally that is also home to thousands of US troops.

Yoon joined Biden and Japan's then prime minister Fumio Kishida in 2023 for a landmark three-way summit at the Camp David presidential retreat that included a promise to step up intelligence cooperation on North Korea.

Choi's office said in a statement that South Korea remained committed to the "principles and agreements from the Camp David summit".

The acting president "stated that South Korea will continue to maintain its diplomatic and security policies based on a strong Korea-US alliance and trilateral cooperation among South Korea, the US and Japan", the statement said.

- Changes expected under Trump -

South Korea's progressive opposition, which has made Yoon's life miserable from parliament and is increasingly ascendant since the president's power grab, historically has taken a harder line on Japan.

Opposition leader Lee Jae-myung -- who himself faces election disqualification in a court case -- also favours more diplomatic outreach with North Korea than the hawkish Yoon.

The turmoil and lack of a clear leader in Asia's fourth-largest economy comes just as the US is in the midst of its own political transition.

While Biden has focused on nurturing US alliances, Trump, who takes over on January 20, has been dismissive of what he sees as unfair commitments by Washington.

Trump said during his latest presidential run that if he were in power he would have strong-armed South Korea into paying $10 billion a year for the US troop presence, nearly 10 times what it contributes now.

But paradoxically, Trump forged a bond with the last progressive South Korean president, Moon Jae-in, who encouraged his attempts at deal-making with North Korea.

Trump, who once threatened "fire and fury" against North Korea, went on to meet three times with leader Kim Jong Un and said they "fell in love".

Trump's unusually personal diplomacy managed to lower tensions on the Korean peninsula but brought no lasting deal to end Pyongyang's nuclear programme.