China’s testing of a hypersonic missile designed to evade American nuclear defenses was “very close” to a “Sputnik moment” for the United States, Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, said Wednesday in the first official confirmation of how Beijing’s demonstration of its weapon capabilities had taken American officials by surprise.

Existing defenses of the continental United States all point west and north over the Pacific, meaning they might fail in defeating an attack from the south. Even if there were antimissile bases pointed south, current antimissile technology is designed to intercept intercontinental ballistic warheads on predictable, parabolic paths in outer space — not hypersonic weapons that can zig and zag through the atmosphere.

“We just don’t know how we can defend against that technology, neither does China, neither does Russia,” said Ambassador Robert A. Wood, who is retiring in a few weeks as the U.S. representative at arms control sessions in Geneva.