More than 1,300 people were reportedly sheltering inside the theater in Mariupol when it was hit by a Russian airstrike last week. The southern Ukrainian city said "about 300" were killed.
Dozens of Ukrainian officials, journalists and activists have been detained or kidnapped by Russian forces, the United Nations has warned.
The UN human rights office (OHCHR) in Kyiv said it had documented the arbitrary detention and enforced disappearance of 22 Ukrainian officials. It said 13 were later released.
The UN body's representative in Ukraine Matilda Bogner said some of the cases resembled "hostage-taking."
She said 15 journalists and civil society activists are among those taken, along with the mayor of Ukraine's southern city of Melitopol, Ivan Fedorov.