Joe Biden and South Korea consult on how to deal with North Korea

Mr Biden told his counterpart at a meeting that their alliance is based on “shared sacrifice” and would be taken to new levels as the emphasis on national security is being augmented by an added focus on trade and technology.

“Our two nations are working to take on both the opportunities and challenges of the moment,” the US president said.

The division of the Korean peninsula after the Second World War has led to two radically different countries.

In South Korea, Mr Biden is touring factories for computer chips and next-generation cars in a democracy and engaging in talks for greater co-operation.

But in the North, there is a deadly coronavirus outbreak in a largely unvaccinated autocracy that can best command the world’s attention by flexing its nuclear capabilities.

Speaking to reporters aboard Air Force One as Mr Biden flew to South Korea, White House national security adviser Jake Sullivan said the US has coordinated with Seoul and Tokyo on how they will respond should the North conduct a nuclear test or missile strike while the US leader is in the region or soon after.

Mr Sullivan also spoke with his Chinese counterpart Yang Jiechi earlier in the week and urged Beijing to use its influence to persuade the North to cease the tests.

“China should contemplate taking whatever steps it can to reduce the possibility of a provocative” act, Mr Sullivan said.

As part of the five-day visit in Asia, Mr Biden is focusing his Saturday on his relationship with Mr Yoon, who assumed office little more than a week ago.

One mission will be reassuring South Korea about the US commitment to countering North Korea’s Kim Jong Un.

There is worry in Seoul that Washington is slipping back to the Obama administration’s “strategic patience” policy of ignoring North Korea until it demonstrates seriousness about denuclearisation, an approach that was criticised for neglecting the North as it made huge strides in building its nuclear arsenal.

Prospects for real nuclear diplomacy are slim as North Korea has ignored South Korean and US offers of assistance with its Covid-19 outbreak, dimming hopes that such cooperation could help ease nuclear tensions or even lead to talks.

Still, Mr Biden and Mr Yoon are expected to discuss ways to work with the international community to get the North much needed vaccines and tests, according to senior Biden administration officials who briefed reporters.

The Biden administration is calling on China to restrain North Korea from engaging in any missile or nuclear tests. Speaking on Air Force One, Mr Sullivan said Mr Biden and Chinese President Xi Jinping could hold a phone call in the coming weeks.

White House officials said Mr Biden will not visit the Demilitarised Zone (DMZ) dividing the Korean peninsula during his trip – something that has become standard for presidents during Seoul visits dating back to Ronald Reagan.

Mr Biden visited the DMZ in 2013 as vice president. Mr Sullivan said the president’s decision to skip the stop this time was not driven by security concerns.

Instead, Mr Biden on Sunday will visit the Air Operations Centre’s Combat Operations Floor on Osan Air Base, south of Seoul. The US sees it as one of the most critical installations in Northeast Asia.

The US president opened Saturday by laying a wreath at Seoul National Cemetery, wearing white gloves and a sombre expression as he also burned incense.

Mr Biden will later meet privately with Mr Yoon, hold a joint news conference and attend a state dinner at the National Museum of Korea.

One focus is sure to be a North that is menacing yet economically fragile. Yet both leaders also are keen to emphasise their growing trade relationship as two Korean industrial stalwarts — Samsung and Hyundai — are opening major plants in the US.

The national security event that is galvanising broader discussions between the two countries has been Russia’s invasion of Ukraine, a war that has led to an unprecedented set of sanctions by the US and its allies.

South Korea joined the US in imposing export controls against Russia and blocking Russian banks from the Swift payments system.

Its participation was key to stopping Russia’s access to computer chips and other technologies needed for weapons and economic development.