"People, children are under the wreckage. Atrocity!" the president tweeted. The port city's deputy mayor also said Russia's military had violated a cease-fire meant to allow civilians to evacuate.

Here's what we're following today:

Russian forces violate a cease-fire: Mariupol's deputy mayor says about 1,200 civilians have been killed in his city in the past two weeks.

Chernobyl site under stress: Ukraine's nuclear agency said the site lost power, and generators only have enough fuel for 48 hours.

Harris to Poland: The vice president travels to Eastern Europe one day after the Pentagon rejected Poland's offer to share its fighter jets with Ukraine.