How is that, I thought US is south Korean ally?
The US doesn't trust Korea 100%, because Korea did have nuke prototypes and were 6 months away from having battle-ready nukes back in 1979 under then President Park Jong Hee. President Park planned to announce his resignation at the event of the announcement of nuclear weapons procession as the sacrificial lamb; he was willing to trade away his power for Korea's right to have nukes. This is the reason why his daughter is expected to win the presidential election in 17 days, because people have a fond memory of his father whom people consider the true patriot.
Unfortunately, Park was assassinated and his successor dictator, the most corrupt president in the past 100 years, traded away the nukes in exchange for the US recognition of his regime. So the US is paranoia about Korea having a secret fusion bomb project somewhere even today, given Korea's highly sophisticated nuclear and fusion power research.