President Barack Obama wrote to Russian President Vladimir Putin to inform him directly that the U.S. government had determined that Russia violated the intermediate-range nuclear treaty, White House spokesman Josh Earnest said Tuesday.
"That is an indication that this is a matter that merits the serious attention of the leaders of both the United States and Russia," Earnest told reporters at a briefing, declining to provide details on how Russia violated the treaty.
Read MoreEU agrees on sanctions against Russia
European Union governments reached a deal on Tuesday to impose economic sanctions against Russia, targeting its oil industry, defense, dual-use goods and sensitive technologies, diplomats said, marking a new phase in the biggest confrontation between Moscow and the West since the Cold War.
The EU sanctions will initially last a year, but will be reviewed after three months, the diplomats said.
The measures will shut state-owned Russian banks out of European capital markets. which could affect European holders of Russian debt and financial services firms that do business there. Other sanction targets include the defense sector and sensitive technologies, including oil, but exclude the vital gas sector, on which Europe is heavily dependent.
Read MoreEU hits Russia with new military, financial sanctions
Also Tuesday, Russia's Foreign Ministry said Foreign Minister Sergey Lavrov and U.S. Secretary of State John Kerry discussed by phone the situation in Ukraine and the Cold War-era arms control deal. Moscow said Lavrov urged Kerry to persuade the Ukrainian government to declare a cease-fire.
—By Reuters with AP and CNBC.com