LONDON — Russian hostilities in Ukraine are preventing grain from leaving the “breadbasket of the world” and making food more expensive across the globe, threatening to worsen shortages, hunger and political instability in developing countries. 

Together, Russia and Ukraine export nearly a third of the world’s wheat and barley and more than 70% of its sunflower oil and are big suppliers of corn. Russia is the top global fertilizer producer. 

World food prices were already climbing, and the war made things worse, preventing some 20 million tons of Ukrainian grain from getting to the Middle East, North Africa and parts of Asia.

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Weeks of negotiations on safe corridors to get grain out of Ukraine’s Black Sea ports have made little progress, with urgency rising as the summer harvest season arrives.

“This needs to happen in the next couple of months (or) it’s going to be horrific,” said Anna Nagurney, who studies crisis management at the University of Massachusetts Amherst and is on the board of the Kyiv School of Economics.