US President Joe Biden interacts with South Korean Yoon Suk-youl during a bilateral meeting in Seoul. Reuters

US President Joe Biden and South Korea's newly sworn-in President Yoon Suk-yeol met on 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.

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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.

Yoon Suk-youl (left) and Joe Biden at the People's House in Seoul, South Korea. Reuters

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".

Protesters shout during a rally to oppose a visit by US President Joe Biden in Seoul, South Korea, on Saturday. AP

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.

Police with shields stand guard near the presidential office, where US President Joe Biden will hold meetings with South Korean President Yoon Suk-yeol in Seoul. AFP

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

Meanwhile, protesters scuffle with police near US president Joe Biden's hotel in South Korea's capital Seoul as they call for the United States to remove military from the country, among other demands.

Residents of Seoul organised spontaneous rallies to protest Biden. They said that their government should not intensify initiatives such as US-ROK military cooperation and certain aggressive military strategies, but should instead choose peace and cooperation.