US President Joe Biden and South Korea's newly sworn-in President Yoon Suk-yeol met Saturday in Seoul to discuss fears of a nuclear weapon test by North Korea, even as the secretive dictatorship battles a raging Covid outbreak.

Biden began his day by paying respects at Seoul National Cemetery, where soldiers killed defending South Korea, including many who fought alongside US troops in the Korean War, are buried.

Wearing white gloves, the US president laid a wreath at the Memorial Tower, which honours tens of thousands of soldiers whose bodies were never recovered.

He then held closed-door talks with Yoon ahead of a joint press conference and state dinner.

On Sunday, Biden, making his first Asia trip as president, travels to another key US ally, Japan.

A US official said that in addition to tensions over North Korea and the US-led campaign to punish Russia for invading Ukraine, Biden's main focus Saturday was establishing "a strong personal relationship" with Yoon, who is less than two weeks into his presidency.

Like Japan, South Korea is seen as a key player in US strategy to contain China and maintain what Washington calls the "free and open Indo-Pacific".

Biden's Asia trip "is about demonstrating unity and resolve and strengthening the coordination between our closest allies", the official told reporters on condition of anonymity. 

However, the visit is overshadowed by what the US official called "sabre-rattling" across the heavily fortified border in North Korea, which the White House believes might use the high-profile moment to test either a nuclear-capable missile or explosive.

South Korean intelligence has also warned that Pyongyang had recently completed preparations for a nuclear test.

"From our standpoint, we are ready," the US official told reporters.

- Cutting-edge investments -
Biden began his trip late Friday by accompanying Yoon on a tour of a massive Samsung semiconductor factory. Relations between the two leaders "got off to a very good start," the US official said.

In his first remarks on arrival, Biden said the US-South Korean alliance is "a linchpin of peace, stability and prosperity" and highlighted the Samsung plant's role in maintaining the fragile global supply chain for semiconductors.

The chips are a vital component in almost every piece of sophisticated modern technology, and South Korea and the United States need to work to "keep our supply chains resilient, reliable and secure", Biden said.

For the US leader, whose Democratic Party fears a possible trouncing in midterm elections in November, snarled supply chains are an acute domestic political challenge, with Americans increasingly frustrated over rising prices and setbacks in the post-Covid pandemic recovery.

Biden emphasised Samsung's decision to build a new semiconductor plant in Texas, opening in 2024.

In the southern US state of Georgia, the governor on Friday announced that South Korean auto giant Hyundai will build a $5.5 billion plant to produce electric vehicles and batteries -- another big priority in Biden's vision for putting cutting-edge technology at the heart of US industrial strategy.

- North Korea Covid outbreak -
Adding to the uncertainty about what is happening in North Korea, the isolated country is going through a major outbreak of Covid-19.

How the crisis impacts Kim Jong Un's decision on nuclear tests is one of the many unknowns that US and South Korean officials are weighing.

"We are very concerned about the Covid situation in the DPRK," the US official said.

"We're very sensitive to the fact that they appear to be facing quite a serious situation. And I think you've seen we stand ready to work with others in the international community as needed to provide assistance."

Former CIA analyst Soo Kim told AFP that North Korea's next step will help steer the US-South Korean relationship under Yoon.

"Should Kim proceed with a test during Biden's visit, he will effectively be helping the two countries find greater justification to work together on the North Korea issue," she said.

In Japan, Biden will meet with Prime Minister Fumio Kishida and the emperor.

On Monday, he will unveil a major new US initiative for regional trade, the Indo-Pacific Economic Framework for Prosperity. A day later, he will join a regional summit of the Quad -- a grouping of Australia, India, Japan and the United States.