Russia 'broke 1987 missile treaty'
The Obama administration has stepped up tensions with Russia, accusing Moscow of conducting tests in breach of a 1987 nuclear missile treaty.
The US has called the breach "a very serious matter", going public with allegations that have simmered for some time.
The treaty confrontation comes at a highly strained time between Barack Obama and Russian president Vladimir Putin over Moscow's intervention in Ukraine and its grant of asylum to National Security Agency whistleblower Edward Snowden.
An administration official said President Obama notified Mr Putin of the US determination in a letter. The finding will be included in a US State Department annual report on compliance with arms control treaties to be released today.
The US says 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 official said the US was prepared to hold top-level discussions on the issue immediately "with the aim of assuring the United States that Russia will come back into compliance with its treaty obligations".
Although the US has raised the issue with Russia in the past through diplomatic channels, it had not previously made the accusation publicly. Russian officials say they have looked into the allegations and consider the matter closed.
In raising the issue now, the US appears to be placing increased pressure on Russia and trying to further isolate it from the international community.
The European Union and the United States plan to announce new sanctions against Russia this week in the face of US evidence that Russia has continued to help separatist forces in Ukraine.
The public finding comes in the wake of congressional pressure on the White House to confront Russia over the claims of cheating on the treaty, which banned all US and Russian land-based ballistic and cruise missiles with ranges between 300 and 3,400 miles.