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Officials are warning that Russian forces are preparing an offensive in Ukraine’s southeast.

While Ukraine has pushed back against Russian troops in recent days – taking back territory, including areas around the capital city of Kyiv – the head of NATO warned that Russia is regrouping and plans to deploy soldiers in eastern and southern Ukraine for a “crucial phase of the war.”

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“Moscow is not giving up its ambitions in Ukraine,” NATO chief Jens Stoltenberg said Tuesday. “We expect a further push in the eastern and southern Ukraine to try to take the entire Donbas and to create a land bridge” to the Crimea Peninsula, which Russia annexed in 2014. Russia-backed separatists in the Donbas have been fighting Ukrainian troops for the last eight years.