CNN's Will Ripley said this case was especially rare because his detention hadn't been reported by state-run media or any other media. Kim was reportedly born in South Korea and moved to the U.S., but has lived in China for the past decade or so.

North Korea detaining foreign visitors, including American citizens, isn't uncommon. The hermit kingdom has detained several Americans for espionage in the past few years. (Video via Arirang)

On Sunday, the U.S. flew a B-52 bomber over South Korean airspace. It was accompanied by South Korean planes and was taken as a display of solidarity with the country against North Korea.

That fly-over came days after North Korea alleged it had tested a hydrogen bomb, although the power of the bomb is disputed.

This video includes images from Getty Images.