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.

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.

The B-52, based in Guam and capable of carrying nuclear weapons, was joined by two fighter planes, a U.S. F-16 and a South Korean F-15, in a low flight over Osan Air Base near Seoul, before returning to Guam, the U.S. military said in a statement. The flight was "in response to recent provocative action by North Korea", it said.

"The United States remains steadfast in its commitment to the defense of the Republic of Korea [South Korea] and to maintaining stability on the Korean Peninsula, to include extended deterrence provided by our conventional forces and our nuclear umbrella," said U.S. Lt. Gen. Terrence O'Shaughnessy.

Experts believe the North's nuclear test, which produced a seismic tremor of 5.1, too small to be a proper hydrogen bomb test, was designed to set the stage for a rare general meeting later this year of its ruling Workers' Party, the first since 1980.

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.

Earlier on Sunday, North Korea's state news agency KCNA quoted Mr. Kim as saying no one had the right to criticise 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 U.S.-led imperialists," it quoted Mr. Kim as saying.

The North's official name is the Democratic People's Republic of Korea.

"It is the legitimate right of a sovereign state and a fair action that nobody can criticise," he said.

Mr. Kim's comments were in line with the North's official rhetoric blaming the United States for deploying nuclear weapons on the Korean peninsula to justify its nuclear programme but were the first by its leader since Wednesday's blast.

The United States has said it has no nuclear weapons stationed in South Korea.

Mr. Kim noted the significance of the timing of the test as being held in the year of the party congress, "which will be a historic turning point in accomplishing the revolutionary cause of Juche," according to KCNA.

Juche is the North's home-grown state ideology that combines Marxism and extreme nationalism established by the state founder and the current leader's grandfather, Kim Il Sung.

KCNA said Mr. Kim made the comments on a visit to the country's Ministry of the People's Armed Forces.