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.

Stoltenberg also insisted that the alliance is standing by to help Ukraine with military equipment, a day after President Volodymyr Zelenskyy appealed for more weaponry.

Zelenskyy addressed the United Nations' most powerful body, the Security Council, on Tuesday, after more grisly evidence emerged of civilian massacres.

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The bodies of at least 410 civilians have been found around Kyiv and a "torture chamber" was discovered in Bucha. 

Satellite imagery from Maxar Technologies showed many of the bodies had been lying in the streets for weeks. The New York Times first reported on the images showing the dead.

President Biden said Russian President Vladimir Putin should be tried for war crimes.

More than 4 million Ukrainians have fled the country since the assault began and more than 7 million have been displaced inside Ukraine, according to the U.N.

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Russia has rejected allegations of atrocities, saying without evidence that the horrific scenes were faked and Moscow said it would speak about Bucha at the U.N. on Tuesday. 

The Associated Press contributed to this report.