The visits to Seoul, followed by Tokyo, are being touted as proof that Washington is seeking to cement its years-long pivot to Asia, where rising Chinese commercial and military power is undercutting decades of US dominance.

However, Biden's first trip as president to the region looks set to be overshadowed by an increasingly belligerent North Korea.

Satellite imagery indicates North Korea is preparing to conduct what would be its seventh nuclear test -- which would cap a record-breaking blitz of launches this year, including intercontinental ballistic missiles.

"North Korea will want to attract global attention by conducting a nuclear test during President Biden's visit," Cheong Seong-chang of the Center for North Korea Studies at the Sejong Institute told AFP.

 Military adjustments

Biden, who will visit some of the nearly 30,000 US troops stationed in South Korea, is ready to make "adjustments" to the US military posture in the region, and Seoul's hawkish new President Yoon Suk-yeol is eager for stronger ties.

Both Biden and Yoon have said they're open for talks with Pyongyang but they expect to see real progress on denuclearisation -- which analysts say is anathema to Kim and will stall talks.

"Biden judges that the North Korean issue can't be resolved through impromptu meetings between the leaders as Trump did," said Woo Jung-yeop, a researcher at the Sejong Institute.

North Korea will be watching the outcome of the Yoon-Biden meeting Saturday very closely, said Yang Moo-jin, a professor at the University of North Korean Studies.

"Depending on the result, North Korea will decide on whether it will speed up or slow down its ICBM and nuclear tests," Yang said.

Sullivan said the security situation regarding North Korea was being "closely" coordinated with South Korea and Japan and that he had also spoken about the issue with his Chinese counterpart on Wednesday.

It is likely that Kim is still debating what to do, in particular due to this US pressure on Beijing -- Kim's sole major ally -- to help rein in Pyongyang's nuclear and ICBM tests, the Sejong Institute's Cheong said.

Kim is also well aware of the gridlock at the UN Security Council after Russia's invasion of Ukraine -- so it is "highly likely" a nuke or ICBM test will come during Biden's Asia tour, Cheong added.

Moscow would likely block any attempt at the UN to impose further sanctions on Pyongyang over a weapons test.

 Strategic neglect?

After Seoul, Biden heads to Japan on Sunday for talks with Tokyo's top leaders, before joining a Quad summit -- a grouping of Australia, India, Japan, and the United States.

Sullivan said Biden is bound for Asia with "the wind at our back" after successful US leadership in the Western response to President Vladimir Putin's now almost three-month-long invasion of Ukraine.

The high military, diplomatic and economic cost imposed on Russia is seen in Washington as a cautionary tale for China, given its stated ambitions to gain control over democratic-ruled Taiwan, even if that means going to war.

But the fact he arrives in Asia under the shadow of a possible North Korean nuclear test is partly due to his "strategic neglect" of the region since he took office, said Park Won-gon, a professor at Ewha University.

Talks between Washington and Pyongyang have been stalled for years, after an extraordinary show of diplomacy between then US president Donald Trump and Kim -- brokered by Yoon's predecessor Moon Jae-in -- ultimately ended in failure.

Trump held three headline-grabbing meetings with Kim and claimed that the two were "in love", but analysts say little to no progress was made in dismantling the North's nuclear programmes.

At a vast military parade in Pyongyang recently, Kim said he was strengthening his nuclear arsenal "at the fastest possible speed".

"In terms of denuclearisation and US-North Korea ties, we have returned to a situation where it's difficult to find any progress," Park said.

"There is no way to really stop North Korea now."