Could Russia use chemical weapons in Ukraine?

"We have serious concerns that Russia may be planning to use chemical or biological agents ... for assassinations as part of a stage or false flag incident, or to support tactical military operations." Those chilling words today from Linda Thomas-Greenfield, the US ambassador to the UN. Ukraine President Zelensky agrees, saying Russian propaganda about Ukraine supposedly developing those types of banned weapons -- with help from the US -- is a sign Moscow might unleash them. Meanwhile, Kyiv today is bracing for a march on the city. CNN international correspondent Matthew Chance brings you the latest from Ukraine's capital.

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