A relentless assault on the Ukrainian port city of Mariupol continued Saturday, as Russian forces shelled the city's downtown, including an area around a mosque that was sheltering more than 80 people — some children.

Ukrainian President Volodymyr Zelenskyy said Russia is bombing Mariupol "24 hours a day." And as Russian units also fanned out to prepare for an assault on Ukraine's capital of Kyiv, Zelenskyy urged his people to continue their resistance, saying Russia would need to carpet-bomb Kyiv and kill its residents to take the city.

"They will come here only if they kill us all," Zelensky...