Ukraine's President Volodymyr Zelenskyy said Russian forces struck a museum in Kupyansk in the east of the country, killing two people, wounding 10 others and burying some under rubble.
Speaking at a U.N. meeting as he sat next to Russian Foreign Minister Sergei Lavrov, Guterres criticized the Kremlin's war in Ukraine, saying it was in violation of the U.N. Charter and international law.
Britain's Defense Ministry said a downward trend of Russian casualties in April is likely accurate after what it described as "exceptionally heavy" losses from January to March this year.
The ministry said it sees "Russian forces are now focused on preparing for anticipated Ukrainian offensive operations."
Elsewhere, Sweden expelled five Russian diplomats for activities it said were "incompatible" with their diplomatic status, while Russia expelled a Moldovan diplomat in what it called a retaliatory move.