Yoon Suk-youl speaks to an official at a bilateral meeting US with President Joe Biden in Seoul, South Korea. File/Reuters

"We are working extraordinarily and intensively with the South Koreans to take the necessary steps to buttress both public perception and the reality of our commitments," the official told Reuters ahead of Yoon's summit with Biden next Wednesday.

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The official said it ranked as one of the greatest US achievements that a number of Indo-Pacific countries that could have built nuclear weapons had chosen not to because of the protection of the so-called US nuclear umbrella.

"We have been very clear that our commitment to that nuclear deterrent stands, is ironclad for South Korea," said the official, who did not want to be identified by name.

Nuclear-armed North Korea is expanding arsenal of missiles and bombs.

"President Biden will ... be talking substantial steps to underscore that, to update it, to make clear that everyone has little doubt of our commitment to standing with South Korea, even in the face of provocation from North Korea, saber rattling from Russia, and frankly ambitions for a nuclear buildup on the part of China," he said.

Yoon's week-long state visit from Monday comes at a time when more South Koreans say their country should develop its nuclear arsenal to guard against attack by nuclear-armed North Korea and its expanding arsenal of missiles and bombs.

The official did not elaborate except to say the moves would involve "a variety of things from certain kinds of computations, more with respect to our actual activities, and some high-level engagements between the United States and South Korea."

Reuters