The head of the Missile Defense Agency says it is only a matter of time before hypersonic weapons are added to the arsenals of America's adversaries.

"The hypersonic threat is real, it is not imagination," Air Force Lt. Gen. Samuel Greaves explained Tuesday at the Capitol Hill Club, noting that defending against hypersonics has become a top priority for the agency.

A hypersonic weapon is a missile that travels at Mach 5 or higher, which is at least five times faster than the speed of sound. This means that a hypersonic weapon can travel about one mile per second.

"Those who have access to the information know that ... the capability to deploy hypersonic weapons has been done. It's real, it's coming, it's a matter of time," Greaves said. "The question will be what have we done to prepare ourselves to mitigate or eliminate that threat five, six, or seven years from now when it shows up," Greaves added.

Greaves comments come amid that assess Russia will be capable of fielding a hypersonic glide vehicle, a weapon that no country can defend against, by 2020.