By Tony Munroe and James Pearson SEOUL (Reuters) - The United States deployed a B-52 bomber on a low-level flight over its ally South Korea on Sunday, in a show of force following North Korea's nuclear test last week. North Korean leader Kim Jong Un maintained that Wednesday's test was of a hydrogen bomb and said it was a self-defensive step against a U.S. threat of nuclear war. North Korea's fourth nuclear test angered both the United States and China, although the U.S. government and weapons experts doubt the North's claim that the device was a hydrogen bomb.