China and Russia have successfully weaponised space by deploying killer satellites with "directed-energy weapons", United States Secretary of Defense Mark Esper has claimed.

Esper issued the stark warning at the Air Force Association's Virtual Air, Space & Cyber Conference on Wednesday (September 16).

He claimed the US's long-standing adversaries "seek to erode our long-standing dominance in air power".

"In space, Moscow and Beijing have turned a once peaceful arena into a war-fighting domain," he said.

"They have weaponised space through killer satellites, directed energy weapons and more in an effort to exploit our systems and chip away at our military advantage.

"Furthermore, our competitors and adversaries exploit cyberspace to undermine our security without confronting our conventional strains.