GENEVA >> As Russia gears up for a military offensive in eastern and southern Ukraine, the United Nations warned Saturday that closures of ports on the Black Sea could trigger a global food catastrophe that yields starvation, mass migration and political instability.

Ukraine’s silos are brimming with grains that in a normal year help feed 400 million people. But the supply chain has been disrupted by the war, with ports unable to operate normally near the heart of a war zone.

“You pull that off the market,” David Beasley, executive director of the U.N. World Food Program, said in a phone call from Ukraine’s western city of Lviv, and “you begin to realize you have a perfect storm coming our way.”

The challenge for global food supplies is keeping open the southern ports of Odesa, Chornomorsk, Yuzhny and Mykolaiv on the Black Sea coast, which are in Ukrainian hands but threatened by Russia’s anticipated offensive.

Beasley said that he had spoken by phone and set out his concerns in writing, but that the Kremlin had not responded to any of the United Nations’ concerns.

“We have made no headway whatsoever,” he said.

“This is where the international community has got to come in and make some very serious decisions about protecting ports for humanitarian purposes and opening up ports for the whole world,” he added, “because the whole world is going to pay a price if we don’t get the ports open.”