“China and Russia made hypersonic weapons a national priority. We didn’t,” William B. Roper, the head of Air Force acquisitions and technology, said on Friday. “Every service now has a major hypersonics program in a departmentwide effort to catch up.”

The United States Air Force has two hypersonic prototypes in testing and while development is on an accelerated pace, the weapons are not scheduled to be operational until 2022. Other parts of the Pentagon, including the Defense Advanced Research Projects Agency, have other hypersonic initiatives, but they are many years down the road.

Still, experts say the threat to the United States appears limited. Russia’s system is being deployed in relatively low numbers, likely no more than a couple of dozen, according to Daryl G. Kimball, the executive director of the Arms Control Association. As a result, the system “does not significantly increase the threat to the United States and the world” of Russia’s already fearsome nuclear arsenal, Mr. Kimball said.

But he said the two countries should discuss hypersonics as part of any new treaty negotiations.

“Washington and Moscow should immediately commence talks on how new weapons technologies and all types of nuclear weapons should be regulated so that neither side believes they can gain an advantage by ‘racing’ ahead of the other,” Mr. Kimball said.

The Russian weapon — known as a hypersonic glide vehicle — can fly lower in the atmosphere, avoiding ballistic missile defense radars. It is mounted on an intercontinental ballistic missile, allowing the warhead to be initially carried toward a target on a traditional piece of technology. But as it gets closer to the target, it is designed to fly at hypersonic speeds in an unpredictable path — making detection, tracking and interception extremely difficult. Most American missile defenses work by predicting the path of an incoming weapon, and shooting an “interceptor” at it.

On Friday, Russia’s defense minister, Sergei K. Shoigu, informed Mr. Putin that the first missile regiment armed with the glide vehicle was operational, the ministry said in a statement. The strategic missile forces chief, Gen. Sergei Karakayev, said at a meeting later in the day that the new missile was deployed with a military unit in the town of Yasny of the Orenburg region on the border with Kazakhstan.

The Pentagon declined to comment on the Russian statement, but other American officials said there was no reason to doubt Moscow had deployed the new weapon.