South Korean President Yoon Suk Yeol and US President Joe Biden visited the Korean War Memorial in Washington on Tuesday ahead of formal White House meetings set to deepen the vital US-South Korean alliance.

Yoon, on a six-day tour of the United States, inspected NASA's Goddard Space Flight Center just outside Washington earlier in the day, along with US Vice President Kamala Harris.

He also laid a wreath at the Tomb of the Unknown Soldier in Arlington National Cemetery before joining Biden at the war memorial, which depicts life-sized steel statues of US soldiers marching during the 1950-53 conflict against the communist North Korean forces.

On Wednesday, Yoon and his wife Kim Keon Hee will arrive at the White House for what is only the second full-fledged state visit of the Biden era, following that of France's Emmanuel Macron.

This will feature lavish ceremonial events and a gala White House dinner.

On Tuesday, the White House said Biden had presented Yoon with a handmade mahogany table and vintage baseball memorabilia. First Lady Jill Biden presented her counterpart with a "pendant necklace with a trio of blue sapphires designed by a Korean American designer."

Beyond the pomp and circumstance, Biden and Yoon are eager to discuss their countries' deepening partnership in an increasingly volatile Pacific, where North Korea is ramping up nuclear-capable missile production and China is saber-rattling around Taiwan.

White House National Security Advisor Jake Sullivan told reporters that Biden and Yoon have had four "engagements" since Yoon took office less than a year ago, and "have developed a rapport."

Washington and Seoul are also highlighting their strong cultural links, a connection underscored by streaming giant Netflix's announcement of a $2.5 billion investment in South Korean content. Netflix CEO Ted Sarandos met with Yoon in Washington on Monday.

- US defense of South Korea -

A priority for Biden will be reassuring his guest over the US commitment to "extended deterrence" -- the US nuclear and military umbrella South Korea falls under.

"President Biden will reinforce and enhance our extended deterrence commitments to South Korea with respect to the threat" from North Korea, Sullivan said.

Sullivan said to expect "major deliverables on extended deterrence, on cyber cooperation and climate mitigation, foreign assistance, on investment, and on strengthening our people-to-people ties."

But the two leaders have some "uncomfortable" topics to discuss, noted Katharine Moon, professor emerita of political science at Wellesley College.

The South Korean president has seen his domestic approval ratings dive over his handling of a recent US intelligence leak that appeared to reveal Washington was spying on Seoul.

However, he told NBC News in an interview that the spat would not have lasting impacts.

"I believe that this matter is no reason to shake the ironclad trust that supports the US-South Korea alliance, because it is based on shared values like freedom," Yoon told NBC News. "When you have that trust, you don’t get shaken."

Yoon is meanwhile likely to come under pressure from the White House to do more to help the US support Ukraine, as Washington looks to South Korea -- the world's ninth-largest arms exporter -- to help secure ammunition and weapons for Kyiv.

South Korea has sent humanitarian assistance to Ukraine, and has sold tanks and howitzers to Poland, but Seoul has a longstanding policy of not providing weapons to active conflict zones.

Seoul is also mired in a diplomatic spat with China after Yoon blamed recent heightened tensions over Taiwan on "attempts to change the status quo by force."