President Joe Biden and his new South Korean counterpart agreed on Saturday to hold bigger military drills and deploy more U.S. weapons if necessary to deter North Korea, while expanding the alliance to tackle new regional and global challenges.

Biden and Yoon Suk-yeol said their countries' decades-old alliance needed to develop not only to face North Korean threats but to keep the Indo-Pacific region "free and open" and protect global supply chains.

The two leaders are meeting in Seoul for their first diplomatic engagement since the South Korean president's inauguration 11 days ago. The friendly encounter between allies is clouded by intelligence showing North Korean leader Kim Jong Un is prepared to conduct nuclear or missile tests.

Yoon had sought more assurances that the United States would boost its deterrence against North Korean threats. In a joint statement, Biden reaffirmed the U.S. commitment to defend South Korea with nuclear weapons if necessary.

The two sides agreed to consider expanding their combined military drills, which had been scaled back in recent years in an effort to lower tensions with the North, the statement said.

The United States also promised to deploy "strategic assets" - which typically include long-range bomber aircraft, missile submarines, or aircraft carriers - if necessary to deter North Korea.

Biden told a joint news conference after their meeting that he would be willing to meet with North Korean leader Kim Jong Un if he seemed interested in serious talks. He said Washington had offered COVID-19 vaccines to North Korea and China.

Yoon has also offered aid for North Korea to combat its first acknowledged COVID-19 outbreak, but so far Pyongyang has not responded.

North Korea reported more than 200,000 new patients suffering from fever for a fifth consecutive day on Saturday, but the country has little in the way of vaccines or modern treatment for the pandemic.

EXPANDING ALLIANCE

The U.S.-South Korea alliance, which dates to the 1950-1953 Korean War, must further develop to keep the Indo-Pacific "free and open", Biden said.

He said the alliance was built on opposition to changing borders by force - an apparent reference to Russia's war in Ukraine and China's claims over Taiwan.

Changes in international trade and supply chains gave new impetus for the two countries to deepen their relationship, Yoon said, calling for cooperation on electric batteries and semiconductors.

Biden used the visit to tout investments in the United States by Korean companies, including a move by South Korea's Hyundai Motor Group to invest about $5.5 billion to build its first dedicated fully electric vehicle and battery manufacturing facilities in the United States.

The two leaders toured a massive Samsung semiconductor plant on Friday, where Biden said countries like the United States and South Korea that "share values" needed to cooperate more to protect economic and national security.

Yoon said the concept of economic security will include cooperating in case of shocks in the foreign exchange market.

The South Korean president, keen to play a bigger role in regional issues, is expected to make his country one of the inaugural members of Biden's Indo-Pacific Economic Framework (IPEF), which will be announced during the trip to set standards on labour, the environment and supply chains.

But, given that China is South Korea's top trading partner, he is likely to strike a cautious tone in public on the topic of explicitly countering Beijing. He said on Friday that South Korea's joining the IPEF did not have to conflict with the countries' economic ties.

Although White House officials have sought to play down any explicit message of countering China, it is a theme of Biden's trip and one that has caught the eye of Beijing.

"Jake Sullivan said that Biden's trip to Asia is not aimed at confronting China," Chinese envoy for Korean affairs Liu Xiaoming, said on Twitter, referring to Biden's national security adviser.

"We hope that the U.S. will match its words with deeds and work with countries in the region to promote solidarity and cooperation in the Asia-Pacific, instead of plotting division and confrontation."