Russia held exercises demonstrating its power to sink ships and stop those that try to run its blockade. For Ukrainian food exports to resume, Moscow said, a list of demands must be met.
Vladimir V. Putin and Recep Tayyip Erdogan have kept good relations despite the war in Ukraine, and analysts say that will continue after the Kremlin rejected a deal Mr. Erdogan helped broker.
Moscow withdrew from a deal that allowed Ukraine to export grain across the Black Sea despite a wartime blockade, and warned that ships sailing around Ukrainian ports would be seen as military targets.