The United States accused Russia of violating a historic nuclear test ban treaty by testing a ground-launched cruise missile.
U.S officials said President Barack Obama informed Russian President Vladimir Putin in a letter Monday of the United States' determination that Russia broke the 1987 treaty.
The move was first reported Monday evening by The New York Times.
The U.S. said Russia tested a new ground-launched cruise missile, breaking the Intermediate-Range Nuclear Forces Treaty that President Ronald Reagan signed with Soviet leader Mikhail Gorbachev.
The Intermediate-Range Nuclear Forces Treaty was designed to eliminate ground-launched cruise missiles with ranges of 500-5,500 kilometers.
The U.S. is calling the matter "very serious" and White House spokesman Josh Earnest on Tuesday said Russia's response to its complaint was not satisfactory.
"We have raised concerns with the Russians about the importance of complying with this aspect of the treaty and, I guess suffice it to say, the response we received from them was unsatisfactory," he said.
Earnest said the U.S. intends to pressure Russia on the issue because it believes compliance with the treaty is in the "clear national security interests" of the United States.
"We're going to hold them to living up to the commitments that they made," he said.
The accusation comes during a time of heightened tension between Washington and Moscow over Russia's support of separatists fighting in eastern Ukraine and its decision to grant asylum to National Security Agency leaker Edward Snowden.
The top diplomats for the two countries, U.S. Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov, discussed the issue in a phone conversation. But the Russian Foreign Ministry did not comment on the U.S. complaint.
U.S. officials said the Obama administration is willing to hold high-level discussions on the issue and wants assurances that Russia will comply with the treaty requirements going forward.
Russia hinted at quiting a year ago
Russia and the United States destroyed nearly 2,700 nuclear-armed missiles as part of a Cold War-era nuclear test ban treaty.
The treaty has no expiration date, but a year ago, Sergei Ivanov, a former Russian defense minister and now Putin's chief of staff, raised the possibility of quitting the treaty. He told a Russian television interviewer that "on the one hand, we've signed the agreement, we will obey it. But that could not last forever."
He said the United States does not need the mid-range missiles because they could only be used to target two of it closest allies, Mexico and Canada.
At the time, Putin said that the land-based cruise missiles were not crucial for the United States, but that "practically all our neighbors develop these systems."
The Russian leader said that for "modern Russia," the decision 27 years ago to end use of the land-based cruise missiles was "disputable."
The Intermediate-Range Nuclear Forces Treaty was signed by U.S. President Ronald Reagan and Soviet leader Mikhail Gorbachev. At the time, it was viewed as a cornerstone for ending the Cold War.
