Two Russian missiles struck Kyiv during a visit by the head of the United Nations, Ukrainian officials said, but the West believes the battles for the besieged port of Mariupol and other areas in the east and south may determine the war’s outcome.

Russia withdrew its forces from outside Kyiv last month after failing to take the capital and launched a massive attack on Ukraine’s eastern Donbas region. But Thursday’s blasts in Kyiv, heard soon after U.N. Secretary General Antonio Guterres’s talks with Ukraine’s President Volodymyr Zelensky, highlighted concerns the capital remains vulnerable.