The US military said a B-52 bomber is deployed on a low-level flight over South Korea on Sunday, in response to the nuclear test conducted by North Korea last week.
North Korea announced on Wednesday morning that a test of a miniaturised hydrogen nuclear device had been successfully accomplished and underlined it as a significant improvement in the isolated state's strike power.
According to a statement released on Sunday by the US Pacific Command, the B-52 bomber which was joined by two fighter jets, a US F-16 and South Korean F-15 flew over the Osan Air Base located in Pyeongtaek, the south of Seoul and roughly 100 km (62 miles) from the North Korean border.
Admiral Harry B. Harris Jr., commander US Pacific Command, said “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.”
North Korea's fourth nuclear test angered both the United States and China, although the US government and weapons experts doubt the North's claim that the device was a hydrogen bomb.
After the North's last test, in 2013, the United States sent a pair of nuclear-capable B-2 stealth bombers over South Korea. At the time, the North responded by threatening a nuclear attack on the United States.
The two Koreas remain in a technical state of war after their 1950-53 conflict ended in a truce, not a peace treaty, and the United States has about 28,500 troops based in South Korea.
North Korean leader Kim Jong Un has maintained that Wednesday's test was a self-defensive step against a US threat of nuclear war.
Earlier on Sunday, North Korea's state news agency KCNA quoted Kim as saying no one had the right to criticize its nuclear tests.
"The DPRK's H-bomb test ... is a self-defensive step for reliably defending the peace on the Korean Peninsula and the regional security from the danger of nuclear war caused by the US-led imperialists," it quoted Kim as saying.