South Korea's Joint Chiefs of Staff says that North Korea has attempted to launch a missile from an eastern coastal city, but the launch appeared to end in failure.
There was a high degree of confidence it was not an intercontinental ballistic missile, a US official said on condition of anonymity.
He said: "We will respond to an all-out war with an all-out war and a nuclear war with our style of a nuclear attack".
The status of the North Korean underground nuclear test program is unchanged, a senior USA defense official told CNN, and a test could come at any time.
White House national security adviser H. R. McMaster said on Sunday that North Korea's missile test was provocative and that the United States was working with its allies, including China, to develop a range of options.
South Korea, which hosts 28,500 USA troops, warned of punitive action if the launch led to further provocations such as a nuclear test or a long-range missile launch.
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The adviser said the missile's flight lasted four or five seconds. "The President has no further comment", US Defense Secretary James Mattis said.
It is thought to have a small arsenal of atomic bombs and an impressive array of short- and medium-range missiles.
China is seeking Russia's help to cool surging tensions over Pyongyang's nuclear ambitions, the country's foreign minister has told his Moscow counterpart, after Beijing warned of possible conflict over North Korea.
North Korea's actions come as tensions on the Korean Peninsula have spiked to alarming levels.
The U.S. nuclear-powered USS Carl Vinson aircraft carrier strike group is also heading to the region.
Analysts warn that even failed missile launches provide valuable knowledge to North Korea as it tries to build its weapons program.
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Saturday's parade combined with yesterday's failed missile launch made a sixth nuclear test increasingly likely, and if one was carried out, China would be compelled to support new sanctions against North Korea, the Global Times, an influential tabloid published by the Chinese Communist Party said in an editorial.
Hours before Pence left, North Korea showed off its ICBMs in a military display at the annual parade in the capital that celebrates the birthday of the North's founding ruler, the grandfather of current leader Kim Jong Un. That was the site of a ballistic missile test earlier this month in which the projectile fell into the Sea of Japan, also known as the East Sea. That launch came shortly before Trump's first meeting with Chinese leader Xi Jinping.
In a live broadcast, a newsreader from North Korea's state-run television service called the new military hardware the country's "modernised strategic missiles".
That missile flew about 60 km (40 miles) but what US officials said appeared to be a liquid-fuelled, extended-range Scud missile only travelled a fraction of its range before spinning out of control.
Despite Sunday's failure, the North's previous claim to have used "standardized" warheads has led to worries that it was making headway in its push to develop small and sophisticated warheads to be topped on long-range missiles.
As concern about the situation mounted, The Sunday Times reported that President Trump's military advisers have assured the United Kingdom that America has the capability to neutralise North Korea's nuclear programme using conventional weapons.
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