WASHINGTON (BLOOMBERG) - The United States is not prepared to counter hypersonic attacks as China and Russia step up their capabilities for the ultra-fast weapons, a top Pentagon official said.

Michael Griffin, the under secretary of defence for research and engineering, said on Tuesday (Nov 13) that the US has not significantly invested in such transformative technology since Ronald Reagan's presidency.

"If the United States is going to remain ahead, we have to reinvest," he said at the Centre for Strategic and International Studies.

"A generation or two has gone by since that era. Other people are not stupid. Other people don't stand still. It's time for us to get back to work."

Hypersonic weapons can travel five times the speed of sound, or Mach 5.

The highly maneuverable and hard-to-detect weapons are among the frontiers where US adversaries are accelerating their capabilities.

If such risks are not managed properly, Griffin said, advances by China and Russia could pose "existential" threats to the US.

American forces need to be "closer to the action," because hypersonic weapons have rapid launch-to-attack intervals that are in the tens of minutes, he said.

In July, the Pentagon informed Congress of its plans for the nation's first hypersonic offensive weapons.

The Defence Department said it would shift US$20 million (S$27 million) in existing funds so the Air Force could accelerate development and deployment of a prototype for a "Hypersonic Conventional Strike" air-launched glide munition, with a first test flight in late fiscal 2020.

An additional US$65 million was sought to accelerate demonstration of a land-launched version.