Russia has violated a key arms control treaty by testing a nuclear cruise missile, the US government says.
Russia tested a ground-launched cruise missile, breaking the Intermediate-Range Nuclear Forces Treaty signed in 1987 during the Cold War, the US said.
A senior US official did not provide further details on the alleged breach, but described it as "very serious".
The bilateral agreement banned medium-range missiles with ranges between 500 and 5,500 km.
The US claims come at a time of heightened tensions between the two sides, with the US criticising Russia for its alleged involvement in the conflict in Ukraine.
"We encourage Russia to return to compliance with its obligations under the treaty and to eliminate any prohibited items in a verifiable matter," the official added.
US President Barack Obama has written to Russian leader Vladimir Putin over the matter, officials say.
This is the first time the US government has made its accusations public, though the issue has simmered for years, the BBC's Paul Blake in Washington reports.
In January, the New York Times reported that US officials believed Russia had began testing ground-launched cruise missiles as early as 2008. The US State Department had said at the time that the issue was under review.
The 1987 treaty is at the heart of American-Russian arms control efforts, and was signed by then-Presidents Ronald Reagan and Mikhail Gorbachev in the final years of the Cold War, our correspondent says.
Also on Monday, Obama spoke to several EU leaders, who agreed on further sanctions on Russia for its role in the Ukraine crisis, the White House said in a statement.
It is thought the latest measures will be far wider than previous sanctions and will target Russian banks, as well as the defence and energy sectors.
Full details are to be finalised at a meeting of EU ambassadors later on Tuesday,
The conflict between the Ukrainian government and pro-Russian rebels, who the US says are supported by Russia, has displaced more than 200,000 people.
Tensions in Ukraine also rose after a Malaysia Airlines plane, flight MH17, crashed in eastern Ukraine, close to the border with Russia.
Western nations have said there is growing evidence that the plane was hit by a Russian-supplied missile fired by rebels. Russia has blamed Ukrainian government forces.