The United Nations said Thursday that it can confirm 726 civilians killed in the first 22 days of Russia's invasion of Ukraine, including 52 children, but the "actual number is likely much higher." Most of the civilian deaths, U.N. Undersecretary General Rosemary DiCarlo said, are from "the use in populated areas of explosive weapons with wide-impact areas," including strikes on apartment buildings, schools, and at least 43 health-care facilities. An American citizen, James Whitney Hill, was killed Thursday in Russian shelling of residential areas in the center of Chernihiv, a city about 90 north of Kyiv, the State Department and Hill's family said. Hill had traveled to Ukraine with his longtime partner, Ira, for treatment of her multiple sclerosis, and he documented on social media the increasing Russian artillery fire on Chernihiv and related hardships endured by residents of the besieged city. "Hill was among dozens of civilians killed by the Russian onslaught Thursday in Chernihiv," CNN reports.  Russian artillery fire on the town of Merefa, outside Kharkiv, killed at least 23 people and injured 26 early Thursday, Ukrainian officials said. The Russian shelling also destroyed a secondary school and a city cultural center, the officials said; The Washington Post was able to verify video of the attack on and damage to the school. "The two high-profile attacks on civilian areas unfolded one day after a Russian airstrike demolished a theater in the port city of Mariupol while hundreds sheltered inside," the Post notes. "Officials on Thursday said some had survived, but they still didn't know how many were killed or injured." The strike on the theater is "another tragedy, in our already mangled Mariupol, of which there is already practically nothing left," Mariupol's mayor said in a video message.