The US military sent a B-52 long-range bomber over the Korean peninsula Sunday in response to a nuclear test announced by North Korea last week.
The bomber flew at low altitude over South Korea's city of Osan, in what the military said was a show of force.
"This was a demonstration of the ironclad US commitment to our allies in South Korea, in Japan, and to the defense of the American homeland," said Admiral Harry Harris Jr, commander of the US Pacific Command forces.
"North Korea's nuclear test is a blatant violation of its international obligations. US joint military forces in the Indo-Asia-Pacific will continue to work with all of our regional allies and partners to maintain stability and security."
North Korea said Wednesday that it had successfully tested a hydrogen bomb for the first time.
The B-52 bomber returned to its base on the US Pacific island of Guam.