What is happening in Ukraine’s Mariupol, the city under Russian siege?

Mariupol resident Sergei Shulgin, 62, stands on May 3 in front of a block of flats heavily damaged by the fighting between Ukrainian and Russian forces. (Alexander Ermochenko/Reuters)

The first civilian evacuees from the battered Mariupol steelworks complex where Ukrainian fighters have made a last stand against Russian forces arrived Tuesday in the southeastern city of Zaporizhzhia.

The 101 civilians were escorted to safety by the United Nations following weeks of negotiations with Russian and Ukrainian forces. They had been holed up in bunkers under the sprawling Azovstal Iron and Steel Works along Mariupol’s waterfront as Russia’s military pummeled the city.

Evacuees from Mariupol steel plant describe brutality of long siege

Last month, Russia declared victory over Mariupol, even as a contingent of Ukrainian troops held out at the plant and refused Moscow’s demand that they lay down their arms. Russian forces now control most of the city, a strategic seaside hub with a prewar population of roughly 450,000.

Officials in Ukraine say that up to 20,000 civilians in Mariupol may have been killed since the start of the Russian invasion in February.

Here’s what to know about the siege of Mariupol — and the humanitarian catastrophe unfolding there now.