The United States has found that Russia violated a 1987 arms control treaty by testing a ground-launched cruise missile, a senior US official said late Monday, calling the matter "very serious."

The announcement adds a new dispute at a time of already heightened tensions between Washington and the Kremlin over the crisis in Ukraine, with western countries accusing Russia of arming Ukrainian separatists and destabilizing the country.

The US concluded in a 2014 report that Russia had violated the Intermediate-Range Nuclear Forces Treaty, which barred it from possessing, producing or flight-testing such cruise missiles with a range of 500 to 5,500 kilometers, the official told AFP.

President Barack Obama has sent a letter to his counterpart Vladimir Putin on the subject, which the administration official described as "a very serious matter which we have attempted to address with Russia for some time now."

Washington was prepared to discuss its determination with Moscow "immediately" in senior-level bilateral talks, the official added, saying Congress and US allies have been kept abreast of the matter.

"The United States is committed to the viability of the INF Treaty," the official said. "We encourage Russia to return to compliance with its obligations under the treaty and to eliminate any prohibited items in a verifiable manner."

The Intermediate-Range Nuclear Forces Treaty (INF), signed by then US president Ronald Reagan and his Russian counterpart Mikhail Gorbachev, eliminated nuclear and conventional intermediate range ground-launched ballistic and cruise missiles.

The official said the INF treaty served the "mutual security interests of the parties" -- not only the United States and Russia but also 11 other successor states of the Soviet Union.

"Moreover, this treaty contributes to the security of our allies and to regional security in Europe and in the Far East."

"The United States will, of course, consult with allies on this matter to take into account the impact of this Russian violation on our collective security if Russia does not return to compliance," the official said.

In January, State Department spokeswoman Jen Psaki said Washington had raised concerns with Moscow following a New York Times report it had begun testing a new ground-launched cruise missile as early as 2008, and that the State Department's senior arms control official had repeatedly raised the issue with Moscow since May 2013.

Psaki said at the time she could not refute the details of the Times report, and that there was an ongoing interagency review to determine whether the Russians had violated the terms of a US-Russian arms control pact.

- Ground-launched cruise missile -

The Times said the State Department's will publish its allegation in its annual report on international compliance with arms control agreements.

"The United States has determined that the Russian Federation is in violation of its obligations under the INF treaty not to possess, produce or flight test a ground-launched cruise missile (GLCM) with a range capability of 500 kilometers to 5,500 kilometers or to possess or produce launchers of such missiles," the Times said the report will say.

Independent Russian military analyst Pavel Felgenhauer told AFP he believes that the US is concerned about a cruise missile version of the Iskander short-range ballistic missile system, the Iskander-K.

He said the missile has been tested at a range of 1,000 km, but could the range could be extended up to 2,000-3,000 km by adding extra fuel tanks.

Felgenhauer noted that Russian leaders were not enthusiastic about the INF treaty.

"Putin and (Putin's chief of staff and former defense minister) Sergei Ivanov have spoken about the INF treaty being detrimental and no longer needed by Russia, with Ivanov referring to the Iskander-K," said Felgenhauer.

jca-jkb-vlk-nss-rl/jm