Secretary of State Rex Tillerson’s team warned Russia that the United States will retaliate against violations of a 1987 arms control treaty by researching the development of a new cruise missile system.

“Despite repeated U.S. efforts to engage the Russian Federation on this issue, Russian officials have so far refused to discuss the violation in any meaningful way or refute the information provided by the United States,” State Department spokeswoman Heather Nauert said Friday.

Nauert issued that message on the 30th anniversary of the INF treaty, which bans the deployment of intermediate-range cruise missiles. U.S. officials have complained since 2014 that Russia is violating the pact by developing — and, in recent months, deploying — a banned cruise missile system. So Congress authorized $50 million to spend on researching programs that could defend against the new cruise missiles, as well as new ground-launched U.S. cruise missiles.

Under Secretary of State Tom Shannon amplified Nauert’s rebuke in a direct appeal to the Russian people.

“[C]ontinuation of a situation in which the United States remains in compliance while Russia violates the agreement is unacceptable to us,” Shannon told Kommersant, a Russian daily newspaper. “This is because Russia is illegally seeking a unilateral military advantage while undermining future arms control engagements.”

Russian President Vladimir Putin’s team has denied any violations of the Cold War-era arms control deal. "Russia has been, remains and will remain committed to all international obligations, including those arising from the INF Treaty," Kremlin spokesman Dmitry Peskov said in March.

Such denials have carried weight with some American analysts, who noted that the original 2014 complaint from the State Department “provides few factual details regarding the alleged violations.” But U.S. military officials have been emphatic as the Russian missile system progressed.

“The system itself presents a risk to most of our facilities in Europe,” Air Force Gen. Paul Selva, the vice chairman of the Joint Chiefs of Staff, told Congress in March. “And we believe that the Russians have deliberately deployed it in order to pose a threat to NATO and to facilities within the NATO area of responsibility.”

Shannon said that the United States would begin its own new military programs and research until and unless Russia dismantled the offending missile system. “Let me be clear: the United States will not take any action that is in violation of our INF Treaty obligations,” he said. “We are prepared to cease such research into INF-prohibited systems if Russia returns to full and verifiable compliance with its INF Treaty obligations.”