Russian troops have ceased making advances on the ground toward the Ukrainian capital of Kyiv and appear to have refocused their aims in eastern Ukraine, a senior U.S. Defense official said Monday.

The Russians appear intent on cutting off Ukrainian forces in the Donbas region, according to the official who discussed intelligence assessments on condition of anonymity. The Russian move could also be aimed at establishing authority there to gain leverage in negotiations for a cease-fire or peace deal, the official said.

Russia has been backing separatists in the Donbas region of eastern Ukraine since 2014. Last week, a high-ranking Russian military official claimed the “main tasks” of the invasion that began Feb. 24 was successfully completed.

“The combat capabilities of the Ukrainian armed forces have been substantially reduced, which allows us to concentrate our main efforts on achieving the main goal – the liberation of Donbas,” Sergei Rudskoy said.

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The Pentagon official, however, said Ukrainian forces continue to thwart Russian efforts to dominate the country’s airspace. Russian pilots continue to show risk-averse behavior, fearing attacks from Ukrainian air defenses. Most of Russia’s air-launched cruise missile attacks have come from aircraft operating from the safety of airspace over Russia or Belarus, the official said.

Ukrainian resistance has held in most major cities against the Russian invasion, the official said. The city of Mariupol in the south, however, continues to face heavy bombardment from Russian forces, and the mayor of Mariupol on Monday called for evacuation of the remaining 160,000 residents.

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