South Korean President Yoon Suk-yeol set off on Monday for the United States and a summit with U.S. President Joe Biden at a time of rare questioning in South Korea of an alliance that has guaranteed its security for decades.

Yoon's April 24-29 trip is the first state visit to the U.S. by a South Korean leader in 12 years and will mark the 70th anniversary of a partnership that has helped anchor U.S. strategy in Asia and provided a foundation for South Korea's emergence as an economic powerhouse.

But as North Korea races ahead with the development of nuclear weapons and missiles to carry them, there are growing questions in South Korea about relying on "extended deterrence," in essence the American nuclear umbrella, and there are even some calls from senior members of Yoon's party for South Korea to develop its own nuclear weapons.