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STORY: "Our mutual defense treaty is ironclad."

President Joe Biden announced an agreement with South Korea on Wednesday that will give the key U.S. ally - for the first time - a bigger role in nuclear weapons planning over any conflict with North Korea.

In a press conference at the White House with South Korean President Yoon Suk Yeol, Biden directed a warning at North Korea, which continues to grow its arsenal of missiles and bombs.

"Look, a nuclear attack by North Korea against the United States or its allies or partisans... or partners is unacceptable and will result in the end of whatever regime were to take such an action."

The announcement includes a renewed pledge by South Korea not to pursue a nuclear weapons program of its own.

Officials said the U.S. will give South Korea more insights into, and a voice in, U.S. contingency planning to deter and respond to any nuclear incident in the region.

As part of the deal, the U.S. will deploy a ballistic-missile submarine to South Korea in a show of force - the first such deployment since the early 1980s.

But Biden made clear no U.S. nuclear weapons would be stationed on South Korean territory.

"I have absolute authority as commander in chief and the sole authority to use a nuclear weapon, but … what the declaration means is that we're going to make every effort to consult with our allies when it's appropriate, if any action is so called for."

The summit in Washington also comes just weeks after the leak of dozens of confidential U.S. documents, one of which gave details of internal discussions among senior South Korean officials about U.S. pressure on Seoul to supply weapons to Ukraine, and its policy of not doing so.

REPORTER: "Did the recent leaks revealing that the U.S. was spying on South Korea come up at all in your discussions?"

YOON: "With regard to that, we are communicating between our two countries now. We are sharing necessary information. I believe that an investigation is underway in the United States, so... various and complex variables are always in play. We need time to wait for the investigation results by the United States and we plan to continue to communicate on the matter."

The talks at the White House also produced agreements on cyber security, electric vehicles and batteries, quantum technology, foreign assistance and economic investment.