North Korea on Monday fired a missile as US Secretary of State Antony Blinken visited South Korea, where he warned Pyongyang was working ever more closely with Russia on advanced space technology.

Blinken's trip came 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.

Blinken, who had planned to encourage South Korea to maintain Yoon's policy of enhancing cooperation with Japan, was in talks in Seoul when North Korea fired a ballistic missile that fell into the sea.

The missile flew around 1,100 kilometres (680 miles), the South Korean military said.

"Today's launch is just a reminder to all of us of how important our collaborative work is," Blinken said, pointing to increased three-way exercises and intelligence-sharing on North Korea.

Blinken and his South Korean counterpart Cho Tae-yul condemned the launch at a joint news conference, where the outgoing US top diplomat warned that Russia was stepping up support for North Korea as a reward for its help fighting Ukraine.

North Korea "is already receiving Russian military equipment and training. Now we have reason to believe that Moscow intends to share advanced space and satellite technology with Pyongyang," Blinken said.

Blinken also renewed concern that Russia, a veto-wielding UN Security Council member, would formally accept North Korea as a nuclear state, which would be a major blow to global consensus that Pyongyang must end its program.

US and South Korean intelligence believe that North Korea late last year sent thousands of troops to fight against Ukraine and has already suffered hundreds of casualties.

In Tokyo, where Blinken heads later Monday, Prime Minister Shigeru Ishiba voiced alarm that North Korea's "technology has been improving."

Changes set under Trump

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

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".

President Joe Biden offered a more traditional formula of working-level talks focused on ending North Korea's nuclear program.

Blinken defended the outgoing administration's approach, saying it had reached out to North Korea and only got "more and more provocative actions, including missile launches". 

The tensions are "not for want of seeking engagement and trying to find a way forward through diplomacy", he said.

Blinken visited by coincidence on the anniversary of the violent January 6 riot at the US Capitol by Trump's supporters, after he refused to accept defeat to Biden in the 2020 presidential election. 

Supporters of Yoon have waved US flags and invoked Trump by waving signs with his slogan "Stop the Steal."

Blinken, whose hotel was within earshot of rowdy protests by supporters and opponents of Yoon, steered clear of taking sides in the bitterly divided country.

He repeated US concern over Yoon's brief imposition of martial law on December 3 and saluted the "democratic resilience" of South Korea, without commenting on attempts to arrest the president.

"The United States has full confidence in South Korea's institutions, and we reaffirm our unwavering support for the Korean people as they work tirelessly to uphold those institutions," Blinken said.

Promises of continuity

Yoon had earlier been a darling of the Biden administration. He was selected to lead a global democracy summit and delighted Washington by moving 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.

South Korea's "foreign policy direction will remain unchanged. I made this clear," Cho told Blinken.

Blinken also met acting president Choi Sang-mok, a technocrat in the role just over a week, whose office said that South Korea remained committed to the "principles and agreements from the Camp David summit" and to a "strong Korea-US alliance."

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.

It has also favored a more diplomatic approach with North Korea than the hawkish Yoon.