China’s hypersonic weapon test in July included a technological advance that enabled it to fire a missile as it approached its target travelling at least five times the speed of sound — a capability no country has previously demonstrated.

Pentagon scientists were caught off guard by the advance, which allowed the hypersonic glide vehicle, a manoeuvrable spacecraft that can carry a nuclear warhead, to fire a separate missile mid-flight in the atmosphere over the South China Sea, according to people familiar with the intelligence.

Experts at Darpa, the Pentagon’s advanced research agency, remain unsure how China managed to fire countermeasures from a vehicle travelling at hypersonic speeds, said the people familiar with details of the demonstration.

Military experts have been poring over data related to the test to understand how China mastered the technology. They are also debating the purpose of the projectile, which was fired by the hypersonic vehicle with no obvious target of its own, before plunging into the water.