The Obama administration has accused Russia of violating a key arms control agreement by testing an intermediate-range nuclear cruise missile.
The tests, which U.S. officials have said began as early as 2008, have been a point of contention between the two countries for at least a year. The U.S. raised the issue with the Russians last year and informed NATO allies of the issue. But it did not formally accuse Russia of violating the 1987 treaty until Monday.
The administration has been debating whether to declare a formal violation for months. Some members of Congress have accused the White House of dragging its feet on the issue.
It is unclear whether the Russians have placed any of the missiles into their arsenals or whether the missile was tested but subsequently not deployed.
An administration official confirmed that the U.S. had formally notified the Russian government Monday in a letter that the missile test violated the treaty. The letter was first reported by the New York Times.
"The United States has determined that the Russian Federation is in violation of its obligations under the Intermediate-Range Nuclear Forces (INF) Treaty," the official said. The treaty requires both sides not to "possess, produce or flight-test a ground-launched cruise missile with a range" of between 300 and 3,400 miles.
The treaty, signed by President Reagan and Soviet leader Mikhail Gorbachev in December 1987, eliminated a class of nuclear missiles from Europe. Russian President Vladimir Putin has publicly questioned the agreement, but has said Russia would abide by it.