The United States flew some of its most advanced warplanes in bombing drills with ally South Korea, a clear warning after North Korea launched a mid-range ballistic missile designed to carry nuclear bombs over Japan earlier this week, the US and South Korean militaries said.

North Korea hates such displays of US military might at close range and will likely respond with fury, AP reports.

Two US B-1B supersonic bombers and four F-35B stealth fighter jets joined four South Korean F-15 fighters in live-fire exercises at a military field in eastern South Korea that simulated precision strikes against the North's "core facilities", according to the US Pacific Command and South Korea's Defence Ministry.

A US Air Force B-1B bomber drops a bomb over South Korea. Photo / AP
A US Air Force B-1B bomber drops a bomb over South Korea. Photo / AP

The B-1Bs were flown in from Andersen Air Force Base in Guam while the F-35Bs came from a US base in Iwakuni, Japan.

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North Korea, which claims Washington has long threatened it by flaunting the powerful US nuclear arsenal, describes the long-range B-1Bs as "nuclear strategic bombers" although the United States no longer arms them with nuclear weapons.

Hours after the announcements by Washington and Seoul, North Korea's official Korean Central News Agency issued a short statement calling the exercises a "rash act of those taken aback" by North Korea's recent missile launch.

Kim's blast came as he ordered his troops to prepare for an "imminent war" with the US.

Officers are ordering their troops to supplement their meagre food rations by plundering local fields, in order to keep up their strength for battle, according to a report in the Daily NK.

"The military officers are instructing their soldiers, exhausted after training, to eat corn in the fields because war is imminent," a source in North Hamgyong Province told the news website.

"They are even threatening their soldiers, saying: if you become malnourished despite permission to eat the corn, you will face difficulties."

The duelling military displays open up the risk that things will get worse as each side seeks to show it won't be intimidated.

 North Korean leader Kim Jong-u n, smiles as Kim inspects the test launch of a missile. Photo / AP
North Korean leader Kim Jong-u n, smiles as Kim inspects the test launch of a missile. Photo / AP

North Korea has made it clear that it sees its weapons programme, which demands regular testing to perfect, as the only way to contest decades of US hostility, by which it means the huge US military presence in South Korea, Japan and the Pacific.

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Washington, in turn, seeks with its joint drills with Seoul and bomber flights to show that it will not be pushed from its traditional role of supremacy in the region.

More missile tests, more bomber flyovers and three angry armies facing each other across the world's most heavily armed border raises the possibility that a miscalculation could lead to real fighting.

The US Pacific Command said the exercises were conducted in response to North Korea's recent missile launch.

Over the course of a 10-hour mission, the B-1Bs, F-35Bs and two Japanese F-15 fighters first flew together over waters near Kyushu, Japan. The US and South Korean warplanes then flew across the Korean Peninsula and participated in the live-fire training before returning to their respective home stations, according to the Pacific Command.

"North Korea's actions are a threat to our allies, partners and homeland, and their destabilising actions will be met accordingly," General Terrence J O'Shaughnessy, commander of the US Pacific Air Forces, said in a statement.

US Air Force B-1B bombers, top, and second from top, and South Korean fighter jets F-15K fly over South Korea. Photo / AP
US Air Force B-1B bombers, top, and second from top, and South Korean fighter jets F-15K fly over South Korea. Photo / AP

"This complex mission clearly demonstrates our solidarity with our allies and underscores the broadening co-operation to defend against this common regional threat. Our forward-deployed force will be the first to the fight, ready to deliver a lethal response at a moment's notice if our nation calls."

In Beijing, North Korea's ally China warned that war is not an option in finding a solution to the North's growing nuclear capabilities.

Defence Ministry spokesman Colonel Ren Guoqiang told reporters that all parties should exercise restraint and avoid words and actions that escalate tension.

The bombing exercise came as the United States and South Korea wrapped up their annual Ulchi Freedom Guardian joint military drills that involved tens of thousands of soldiers. North Korea condemns the annual US-South Korea war games as rehearsals for an invasion and described Tuesday's missile launch over Japan as a response to the drills. Washington and Seoul faced calls to postpone or downsize this year's drills.

The United States often sends its warplanes to South Korea, mostly for patrols, when animosity rises on the Korean Peninsula - which is technically in a state of war because the 1950-53 Korean War ended with an armistice, not a peace treaty.

North Korea on Tuesday flew a potentially nuclear-capable Hwasong-12 intermediate range missile over northern Japan and later called it a "meaningful prelude" to containing the US territory of Guam.

North Korean leader Kim Jong Un said the launch was a "curtainraiser of its resolute countermeasures" against the US-South Korea war games and called for his military to conduct more ballistic missile launches targeting the Pacific Ocean.

Bombs dropped by US Air Force B-1B bombers, F-35 stealth fighter jets and South Korean F-15 fighter jets hit a simulated target as they fly over South Korea. Photo / AP
Bombs dropped by US Air Force B-1B bombers, F-35 stealth fighter jets and South Korean F-15 fighter jets hit a simulated target as they fly over South Korea. Photo / AP

North Korea has been maintaining a torrid pace in weapons tests this year as it openly pursues an arsenal of nuclear-armed, intercontinental ballistic missiles capable of reaching deep into the US mainland.

Experts say Kim wants a real nuclear deterrent against the United States to ensure the survival of his government and likely believes that it will strengthen his negotiating position when North Korea returns to talks.

North Korea had earlier threatened to fire a salvo of Hwasong-12s toward Guam, which is home to key US military bases and strategic long-range bombers the North finds threatening. It also flight-tested a pair of developmental ICBMs in July.

South Korean analysts said North Korea's threat against Guam and the launch over Japan on Tuesday are likely attempts to make launches over Japan an accepted norm and win itself greater military space in a region dominated by enemies.

The US and South Korean militaries say the Hwasong-12 fired over Japan's northern island of Hokkaido flew about 2700 kilometres. South Korean Vice Defence Minister Suh Choo-suk told politicians on Thursday that North Korea might have fired the missile at about half its maximum range.

- AP