U.S. President Joe Biden agreed Wednesday with his South Korean counterpart Yoon Suk Yeol on plans for Washington to send nuclear submarines to its major ally for the first time in four decades as part of efforts to bolster extended deterrence measures in response to North Korea's growing nuclear threat.

Biden and Yoon, who is on a state visit marking the 70th anniversary of the bilateral security alliance, released a joint declaration to make unequivocal the U.S. commitments to South Korea's defense, including enhancing military training and forming a group of senior officials to discuss nuclear issues.

At a press conference with Biden after their summit at the White House, Yoon praised the elevation of Washington's extended deterrence to a "new level" and the forthcoming expansion of the mutual security treaty to the realms of cyberspace and outer space.

"Our two countries have agreed to immediate bilateral presidential consultations in the event of North Korea's nuclear attack and promised to respond swiftly, overwhelmingly and decisively using the full force of the alliance including the United States' nuclear weapons," he said.

U.S. President Joe Biden shakes hands with South Korea's President Yoon Suk Yeol as they meet in the Oval Office of the White House on April 26, 2023, in Washington. (AP/Kyodo)

In a fresh warning to North Korea, Biden said a nuclear attack against the United States or its allies is "unacceptable and will result in the end of whatever regime were to take such an action."

The U.S. measures, centered on shielding allies with its so-called nuclear umbrella, were augmented at a moment of rising concern over national security and in the face of a more assertive China, which has recently intensified military pressure on Taiwan.

China on Thursday expressed its "firm opposition" to the U.S.-South Korea joint declaration, with Foreign Ministry spokeswoman Mao Ning saying it would "heighten tensions" on the Korean Peninsula, undermine regional peace and stability and run counter to a denuclearization goal.

Mao told a press conference in Beijing that issues related to the Korean Peninsula are "complicated and sensitive" and urged all countries to play a constructive role for the political settlement of the issue rather than stoking up tensions and confrontations.

The South Korean leader, who took office last May, repeatedly said the alliance is guided by "universal values" such as freedom and democracy, and it will contribute to the peace and prosperity of the Indo-Pacific and beyond.

A day after formally announcing his bid for re-election in 2024, Biden also hailed Yoon's "political courage" to improve South Korea's ties with Japan after many years of tension over historical issues.

The presidents said they agreed to continue efforts toward strengthening trilateral cooperation with Japan.

Under the "Washington Declaration," issued along with a joint statement, they also reaffirmed South Korea's non-nuclear status, abiding by all the obligations under the Non-Proliferation Treaty.

The declaration said South Korea is backed by "the full range of U.S. capabilities" and Washington will "further enhance the regular visibility of strategic assets to the Korean Peninsula."

The assets include nuclear ballistic submarines that will be sent to visit South Korea, something that has not taken place since the early 1980s.

Biden, however, said no nuclear weapons will be stationed on the peninsula, stressing, "We are not walking away from that."

U.S. President Joe Biden (2nd from R) and first lady Jill Biden (R) welcome South Korean President Yoon Suk Yeol and South Korean first lady Kim Keon Hee to the White House during an arrival ceremony on April 26, 2023, in Washington, DC. (Getty/Kyodo)

In an attempt to better understand each other's views, a Nuclear Consultative Group, modeled after U.S. engagement with European allies at the height of the Cold War, will be established at the assistant secretary level.

The main purpose of the group is to provide Seoul with additional insight into how Washington thinks about planning for major nuclear contingencies, according to senior Biden administration officials.

The officials suggested ahead of the summit that the declaration, which was discussed "for months," is aimed at both providing better deterrence against North Korea and helping to quell persistent calls in South Korea for nuclear armament.

In addition to bilateral issues, the summit's joint statement said Biden and Yoon reiterated the importance of maintaining peace across the Taiwan Strait, boosting cooperation with Southeast Asian and other countries in the region, and supporting Ukraine in its defense against Russia's aggression.

Apparently with China in mind, the statement said the nations are "strongly opposed any unilateral attempts to change the status quo in the Indo-Pacific, including through unlawful maritime claims."

China reacted sharply to the two leaders' reference on the Taiwan issue, reiterating its claim that the territory, which Beijing regards as its own, is "at the core of China's core interests" and allows "no interference by any force."

China urges the United States and South Korea "to see clearly the nature of the Taiwan question, follow the one-China principle, be prudent in words and deeds and avoid going further down the road on a dangerous path," Mao said.

Yoon's state visit, the first to the United States by a South Korean leader since 2011, comes as he seeks to forge closer ties with the United States and Japan to advance economic security among other goals.

Amid China's increasing influence in high-tech sectors, the presidents agreed to ensure more resilient supply chains for semiconductors and other vital industrial materials.

Yoon said they have agreed to launch a dialogue for next-generation technologies between their national security councils and pledged to further promote people-to-people exchanges, especially between students, to build a brighter future.

Yoon is due to deliver a speech to a joint session of the U.S. Congress on Thursday.

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U.S., S. Korean leaders to agree on stronger extended deterrence