A White House official said Russia has violated a 30-year-old treaty by deploying a cruise missile.
The two countries agreed in 1987 under the Intermediate-range Nuclear Forces treaty to not possess, produce or flight-test any ground-launched missiles that could fly between 500 and 5,500 kilometers.
In 2014, the U.S. State Department complained that Russia was in violation because it was developing and testing such a missile. Russia denied the allegation.
The White House official on Tuesday confirmed that Russia had progressed to deploying the missile.
The New York Times, which first reported the deployment, said that Russia has one battalion of the banned cruise missiles at a test site at Kapustin Yar, and that another was moved in December to an operational site in another part of the country.
The State Department would not confirm the reported deployment, but spokesman Mark Toner did say that Russia was in violation of the treaty, citing the department's earlier allegations.
In Moscow, presidential spokesman Dmitry Peskov said Russia "remains committed to its international commitments, including to the treaty in question."
U.S. Senator John McCain, who chairs the Senate Armed Services Committee, said Russian President Vladimir Putin is "testing" U.S. President Donald Trump's administration. He also said Congress has made it clear Russia's treaty violations require "a meaningful response."
"In light of the most recent developments, it is time for the new administration to take immediate action to enhance our deterrent posture in Europe and protect our allies," McCain said in a statement. "More broadly, we must continue the ongoing modernization of U.S. nuclear forces and ensure that NATO's deterrence forces are survivable, well-exercised, and increasingly ready to counter Russian nuclear doctrine, which calls for the first use of nuclear weapons."