What is happening in Mariupol, the Ukrainian city under Russian siege?

Moscow issued an ultimatum March 20: Ukrainian forces had to leave the besieged city of Mariupol or face further assaults and a “military tribunal.”

Ukraine flatly rejected the order to surrender the city, which has been battered by a Russian blockade and bombardments for nearly three weeks.

Here’s what to know about Mariupol and the humanitarian catastrophe happening there.