The United States deployed a B-52 bomber on a low-level flight over its ally South Korea yesterday, in a show of force after North Korea's nuclear test last week.
North Korean leader Kim Jong Un maintained last week's test was of a hydrogen bomb and a defensive step against a United States threat of nuclear war.
North Korea's fourth nuclear test angered both the United States and China, although the US Government and weapons experts doubt 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 US F-16 and a South Korean F-15, in a low flight over Osan Air Base near Seoul, before returning to Guam, the US military said. The flight was "in response to recent provocative action by North Korea".
Experts believe the nuclear test, which produced a seismic tremor of 5.1 on the Richter scale, too small to be a proper hydrogen bomb, was designed to set the stage for a rare general meeting later this year of North Korea's ruling Workers' Party, the first since 1980.
North Korea will read the fly-over of a bomber capable of delivering nuclear weapons as a threat. Any hint of America's nuclear power enrages Pyongyang, which links its own pursuit of atomic weapons to what it sees as past nuclear-backed moves by the United States to topple its Government.
"This was a demonstration of the ironclad US commitment to our allies in South Korea, in Japan, and to the defence of the American homeland," said Admiral Harry Harris, commander US Pacific Command. "North Korea's nuclear test is a blatant violation of its international obligations."
After North Korea's last test, in 2013, the United States sent a pair of nuclear-capable B-2 stealth bombers over South Korea. At the time, North Korea 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.
The B-52 flight follows a victory tour by Kim to celebrate his country's bomb test. Kim's first public comments about the test came in a visit to the country's military headquarters, where he called the explosion "a self-defensive step" meant to protect the region "from the danger of nuclear war caused by the US-led imperialists".
"It is the legitimate right of a sovereign state and a fair action that nobody can criticise," Kim said.
The tone of Kim's comments provided insight into North Korea's long-running argument that it is the presence of tens of thousands of US troops in South Korea and Japan, and a "hostile" US policy that seeks to topple the government in Pyongyang, that make North Korea's pursuit of nuclear weapons necessary.
During his tour, Kim posed for photos with leading military officials in front of statues of the two members of his family who led the country previously - Kim Jong Il and Kim Il Sung. The United States has said it has no nuclear weapons in South Korea.
South Korean troops, near about 10 sites where loudspeakers started blaring propaganda after the test, were on the highest alert, but had not detected any unusual movement from North Korea along the border, said an official from Seoul's Defence Ministry, who refused to be named.
South Korea's Yonhap news agency said Seoul had deployed missiles, artillery and other weapons systems near the border.
The ministry would not confirm the report, nor another by Yonhap that said North Korea had started its own broadcasts, likely to prevent its soldiers from hearing the South Korean messages.
Officials say broadcasts from the South's loudspeakers can travel about 10km during the day and 24km at night.
That reaches many of the huge force of North Korean soldiers stationed near the border, as well as residents in border towns such as Kaesong, where the Koreas jointly operate an industrial park that has been a valuable cash source for the impoverished north.
• First deployed: 1955
• Length: 47.8m
• Wingspan: 55.5m
• Speed: 1,045.85 km/h
• Range: 14,159km without refuelling
• Armaments: 31,500kg of ordnance, which can include bombs, mines and missiles
• Crew: Five
• Inventory: 58 active, 18 reserve
• Cost: US$84 million