The U.S. ambassador to the United Nations says Russia is "training, arming, bankrolling, and fighting alongside" pro-Moscow rebels battling government forces in eastern Ukraine.
In an emotional June 11 address to an audience in Kyiv, Samantha Power accused Russia of spreading "outright lies" about the Ukraine conflict intended to hide its involvement in a war she portrayed as aimed at thwarting Kyiv's alliance with the West.
She acknowledged that Ukraine faced problems even "before Russia began training, arming, bankrolling, and fighting alongside its separatist proxies in eastern Ukraine, something the Kremlin continues to deny."
Lauding Ukraine's record on media freedoms and transparency in the face of its conflict with Russia, Power accused Moscow of fomenting unrest in Ukraine in order to derail reform efforts after last year's Maidan street protests that helped oust former Ukrainian President Viktor Yanukovych.
"It seems the Russian government's cynical calculation in fueling this aggression was that a new Ukrainian government and newly empowered Ukrainian civil society would be so consumed by the military threat to their nation and so burdened by its destabilizing effects that they would not be able to focus on carrying out the project of the Maidan," Power said.
The Kremlin denies it is backing the separatists in eastern Ukraine despite evidence of direct military involvement, and Russian officials characterize the Maidan movement and Yanukovych's ouster as a Western-backed "coup."
Power said the United States remained supportive of a Minsk cease-fire deal that has been shaky since its inception in February, with the heaviest fighting in months breaking out in Ukraine's east last week between Ukrainian government forces and the Russia-backed rebels.
She said earlier that most violations had been carried out by separatists and by the reappearance on the battlefield of heavy weapons, which had previously been withdrawn.
Monitors from the Organization for Security and Cooperation in Europe (OSCE) have accused both Kyiv and separatist fighters of breaching the cease-fire, which required the withdrawal of heavy weapons from the so-called line of contact.
Power's trip to Kyiv came as two Ukrainian soldiers and three civilians were killed in separate incidents in eastern Ukraine.
More than 6,400 people have been killed since fighting erupted in Ukraine's east in April 2014, one month after Russia seized Crimea from Ukraine.
Moscow has portrayed U.S. Secretary of State John Kerry's meeting with Russian President Vladimir Putin in Russian last month as a sign of Washington's interest in overcoming the impasse over Ukraine.
But the United States and the European Union have continued to deliver sharp public criticism of Moscow and stress the importance of maintaining sanctions against Russia as punishment for the Kremlin's interference in Ukraine.
Power said that Washington will "continue to raise the costs" for Russia if it "continues to disregard the sovereignty and territorial integrity of Ukraine" and "continues to violate the rules upon which international peace and security rest."
Power also said Washington would continue to advocate for the release of Ukrainian pilot and parliament member Nadia Savchenko, who has been jailed in Russia since July.
Savchenko says she was forcibly brought into Russia after being abducted by separatists in eastern Ukraine. She is charged with complicity in the killing of two Russian journalists who died in the Ukraine conflict, as well as illegal border crossing. She denies her guilt and says Russia has no right to detain her.
"The United States will continue to press tirelessly for Nadia's release, as well as the release of all Ukrainians who are being held illegally by the so-called separatists and by Russia," Power said.
With reporting by AFP and Reuters