"Even as Ukraine is building a peaceful, democratic, independent nation across 93 percent of its territory, Crimea and parts of eastern Ukraine are suffering a reign of terror," Assistant Secretary of State Victoria Nuland told the Senate Foreign Relations Committee.
"Today Crimea remains under illegal occupation and human rights abuses are the norm, not the exception, for many at risk groups there," she said of the Black Sea peninsula annexed by Russia a year ago.
Meanwhile, in eastern Ukraine, where a ceasefire between government forces and pro-Russian separatists is largely holding, "Russia and its separatist puppets" had unleashed "unspeakable violence", Nuland said.
Nuland called the insurgency a "manufactured conflict" directed by the Kremlin, which had sent "hundreds of young Russians ... To fight and die there."
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Her broadside came as British Foreign Secretary Philip Hammond said Moscow's "aggressive behaviour" showed it had "the potential to pose the single greatest threat" to British security.
NATO forces are preparing for a major exercise in the Baltic states of Estonia, Latvia and Lithuania, on Russia's western doorstep.
US military officials said the deployment of some 3,000 troops had begun for the three-month Operation Atlantic Resolve, following the Russian annexation of Crimea.
The move came as Russia said yesterday it was quitting a European arms control agreement seen as a cornerstone of security in post-Cold War Europe.