KYIV, Ukraine — Russian strikes hit the city of Zaporizhzhia, the only one of four regions where Kremlin-backed authorities are staging referendums on the prospect of joining Russia in which the regional capital is not under military occupation. Voting is underway in occupied areas of Ukraine, including the Donetsk, Luhansk and Kherson regions, as Russia’s foreign minister insisted that these regions would be “under the full protection of the state” if they are annexed — despite widespread condemnation. Some residents called it a vote “under a gun barrel,” with the outcome predetermined by the Kremlin.