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

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.

Warning that relations with the U.S. are “on the verge of a breach,” Russia has summoned the U.S. ambassador in an official protest against President Joe Biden’s criticism of Russian President Vladimir Putin.

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 nearly 3.4 million people have fled Ukraine.

Here are some key things to know about the conflict:

WHAT IS THE LATEST IN MARIUPOL?

The key port city has seen some of the heaviest fighting since the Russian invasion. Russian and Ukrainian soldiers are fighting block-by-block for control of the city where at least 2,300 people have died, some buried in mass graves.

Ukrainian officials rejected a Russian offer that its troops be granted safe passage out of the encircled city, which would hand Mariupol to Russia, allowing Russian forces in southern and eastern Ukraine to unite.

“There can be no talk of any surrender, laying down of arms,” Ukrainian Deputy Prime Minister Irina Vereshchuk told the news outlet Ukrainian Pravda.

It was not clear how many casualties there were in the Russian bombing of the art school, Zelenskyy said in a video address early Monday.

The strike was the second time in less than a week that officials reported an attack on a public building where Mariupol residents had taken shelter. Last Wednesday, a bomb hit a theater...