Washington (AFP) - Even as the Pentagon hustles to ensure that its defenses keep pace with North Korea's fast-growing rocket program, US officials increasingly are turning attention to a new generation of missile threat.

These weapons under development by China and Russia -- as well as by the United States -- can fly at many times the speed of sound and are designed to beat regular anti-missile defense systems.

The hypersonic missiles could change the face of future warfare, as they can switch direction in flight and do not follow a predictable arc like conventional missiles, making them much harder to track and intercept.

"China's hypersonic weapons development outpaces ours... we're falling behind," Admiral Harry Harris, who heads the military's Pacific Command, warned lawmakers on Wednesday.

"We need to continue to pursue that and in a most aggressive way in order to ensure that we have the capabilities to both defend against China's hypersonic weapons and to develop our own offensive hypersonic weapons," he added.

In its proposed $9.9 billion requested budget for 2019, the Missile Defense Agency (MDA) is asking for $120 million to develop hypersonic missile defenses, a big increase from the $75 million in fiscal 2018.

MDA Director of Operations Gary Pennett told Pentagon reporters this week that the potential deployment by America's rivals of hypersonic weapons -- which could be launched from planes, ships or submarines and carry either nuclear or conventional payloads -- would create a "significant" gap in US sensor and missile interceptor capabilities.

"The key challenge to US national security and the security of US friends and allies is the emergence of new threats designed to defeat the existing" ballistic missile defense system, Pennett said.

So, why the sudden alarm?

According to reports in the Japan-based Diplomat magazine, China has developed -- and last year tested -- a new type of hypersonic missile called the DF-17. The US Office of the Director of National Intelligence this week stated China "has tested a hypersonic glide vehicle."

Russia too is believed to be developing its own hypersonic weapon called the Zircon. According to Russian news agency Tass, it is to go into serial production this year.

- A mile a second -

Though the Pentagon is warning about hypersonics, the United States has been developing the technology for years.

The Air Force says its X-51A Waverider cruise missile, tested in 2012, could travel at speeds faster than Mach 6 (3,600 miles per hour, 5,800 kilometers per hour).

That's more than one mile a second, and future iterations are expected to go much faster.

Part of the reason China has been able to advance its hypersonic missile programs is that it is not subject to anti-missile treaties signed between the United States and Russia.

The 1987 Intermediate Nuclear Forces Treaty banned short- and intermediate-range ground-launched missiles.

"Over 90 percent of China's ground-based missiles would be excluded by INF if they were now in it," Harris said.

Still, by far the lion's share of the MDA's budget continues to go towards improving existing missile-defense systems.

Various sensors and radars can track an incoming missile hurtling towards a target, then blast interceptor rockets toward it to pulverize it with kinetic energy.