Russia said its forces were "tightening the noose" around the besieged Ukrainian port of Mariupol yesterday and concern grew over mass civilian casualties as the United States again warned China against aiding Moscow in its invasion.

Russia's advance in Ukraine has largely stalled, and its troops, frustrated by fierce Ukrainian resistance, have blasted residential areas to rubble. Yesterday, missiles landed near Lviv, a western city where thousands have fled for refuge.

In Mariupol, the scene of heavy bombardment, officials estimated...