Beijing's main advantage is "mass: More ships. More missiles. More people," Deputy Secretary of Defense Kathleen Hicks said at a military technology conference in Washington.

"We'll counter the PLA's mass with mass of our own, but ours will be harder to plan for, harder to hit, harder to beat," she said, referring to the Chinese military.

The goal is "to field attritable autonomous systems at scale of multiple thousands, in multiple domains, within the next 18 to 24 months," Hicks said.

The United States says China is its consequential challenge and is seeking to deter conflict with Beijing, especially over the democratic self-ruled island of Taiwan.

China claims Taiwan as its own and has long vowed to take the territory one day, by force if necessary.