A senior U.S. official said Friday that President Joe Biden would pledge “substantial” actions to deter a North Korean nuclear assault on South Korea at a summit with Yoon Suk Yeol next week.

“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 person told Reuters before Yoon’s summit with Biden next Wednesday.

The official called it one of the greatest U.S. achievements because several Indo-Pacific countries that could have built nuclear weapons chose not to due to the U.S. nuclear umbrella.

“We have been very clear that our commitment to that nuclear deterrent stands, is ironclad for South Korea,” the unnamed person said.

“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 begins Monday as more South Koreans say their country should develop its nuclear arsenal to defend against North Korea’s rockets and bombs.

The official only said the steps would encompass “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.”

The Asan Institute for Policy Studies in Seoul polled South Koreans on April 6 and found 64.3% backed nuclear weapons development and 33.3% opposed it.

52.9% of South Koreans believed the U.S. would defend them with nuclear weapons if North Korea attacked. However, the percentage plummeted to 43.1% when asked if the U.S. would risk its safety to defend South Korea, with 54.2% saying no.

Another official said the US appreciated South Korea’s assistance for Ukraine and would “welcome additional steps (It) might be willing to take.”

“But we also recognize like every country they have to make those decisions based on their own calculations,” he said.

The person said the summit would likely tackle Yoon’s priority, North Korea’s human rights issue.

In an interview with Reuters on Tuesday, Yoon hinted that his government might not “insist only on humanitarian or financial support” if a large-scale attack on civilians or a “situation the international community cannot condone” occurred.

The first official said the summit, just the second state visit under the Biden administration, showed Biden’s admiration of Yoon’s strong leadership and rapprochement with Japan, another major U.S. ally in northeast Asia.

The person said Biden would praise South Korean tech investment in the US, which has approached $100 million since he took office.