Russia-Ukraine war: Key things to know about the conflict

The battle for the strategic Ukrainian port city of Mariupol is raging on

The battle for Ukraine's strategic port of Mariupol raged on Monday, as Ukraine rejected a Russian offer to evacuate its troops from the besieged city and Russian bombardment continued.

Ukrainian President Volodymyr Zelenskyy said about 400 civilians were taking shelter at an art school in the Azov Sea port city when it was struck by a Russian bomb.

Russia's war in Ukraine, now in its 26th day, shows no signs of abating. The invasion has wreaked devastation and destruction, exacting a heavy toll on civilians. The U.N. says more than 3.38 million people have fled Ukraine.

Here are some key things to know about the conflict: