Key points:
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Hypersonics move at speeds of more than five times the speed of sound, or about 6,200 kph
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Sources say the test involved a weapon that first orbited the earth, something called 'fractional orbital bombardment'
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China has denied the claims, saying what it tested was a space vehicle
The Pentagon has been at pains to avoid direct confirmation of the Chinese test this summer, first reported by the Financial Times, even as President Joe Biden and other officials have expressed general concerns about Chinese hypersonic weapons development.
But General Milley explicitly confirmed a test and said that it was "very close" to a Sputnik moment – referring to Russia's 1957 launch of the first man-made satellite, which put Moscow ahead in the Cold War-era space race.
A hypersonic missile blasts off from land. China's hypersonic missile, similar to the one pictured, circled the Earth in low orbit in a test.(KCNA/Reuters)
"What we saw was a very significant event of a test of a hypersonic weapon system. And it is very concerning," Mr Milley, chairman of the Joint Chiefs of Staff, told Bloomberg television, in an interview aired on Wednesday.
Nuclear arms experts said China's weapons test appeared to be designed to evade US defences in two ways.
First, hypersonics move at speeds of more than five times the speed of sound, or about 6,200 kph, making them harder to detect and intercept.
Second, sources tell Reuters that the United States believes China's test involved a weapon that first orbited the Earth.
That's something military experts said was a Cold War concept known as "fractional orbital bombardment."
Last month, Air Force Secretary Frank Kendall alluded to his concerns about such a system, telling reporters about a weapon that would go into an orbit and then descend on a target.
"If you use that kind of an approach, you don't have to use a traditional ICBM trajectory – which is directly from the point of launch to the point of impact," he said.
"It's a way to avoid defences and missile warning systems."
Fractional orbital bombardment would also be a way for China to avoid US missile defences in Alaska, which are designed to combat a limited number of weapons from a country like North Korea.
Jeffrey Lewis at the Middlebury Institute of International Studies summed up fractional orbital bombardment this way:
"The simplest way to think about China's orbital bombardment system is to imagine a space shuttle, put a nuclear weapon into the cargo bay, and forget about the landing gear," Mr Lewis said.
He said the difference was that the Chinese re-entry system was a glider.
Hypersonic missile test reports have China watchers on high alert.
China's foreign ministry denied a weapons test.
China denies it tested a hypersonic missile, like the ones pictured here, saying it was a space vehicle.(Reuters: Thomas Peter/File photo)
It said it had carried out a routine test in July, but added: "It was not a missile, it was a space vehicle."
US defences are not capable of combating a large-scale attack from China or Russia, which could overwhelm the system.
But the open US pursuit of more and more advanced missile defences has led Moscow and Beijing to examine ways to defeat them, experts said, including hypersonics and, apparently, fractional orbital bombardment.
The United States and Russia have both tested hypersonic weapons.
Reuters