US Secretary of State Antony Blinken accused Russia on Thursday of using food as a weapon in Ukraine by holding "hostage" supplies for not just Ukrainians, but also millions around the world.
A senior official in Moscow later rejected the allegations, saying Russians were "not idiots" and would not export food while being subject to tough sanctions.
Addressing the United Nations Security Council, Blinken appealed to Russia to stop blockading Ukrainian ports.
"The Russian government seems to think that using food as a weapon will help accomplish what its invasion has not – to break the spirit of the Ukrainian people," he said.
"The food supply for millions of Ukrainians and millions more around the world has quite literally been held hostage."
The war in Ukraine has caused global prices for grains, cooking oils, fuel and fertiliser to soar.
Russia and Ukraine together account for nearly a third of global wheat supplies. Ukraine is also a major exporter of corn, barley, sunflower oil and rapeseed oil, while Russia and Belarus - which has backed Moscow in its war in Ukraine – account for more than 40% of global exports of potash, a crop nutrient.
Former Russian President Dmitry Medvedev, writing on the Telegram messaging app, said Russians were skilled at producing food needed throughout the world under the right circumstances.
"Everything turns out to be illogical – on the one hand, crazy sanctions are introduced while on the other hand there are demands to supply food," wrote Medvedev, now deputy chairman of Russia's Security Council.
"Things don't work like that. We are not idiots."
Medvedev said producing harvests required people skilled in agriculture, as well as proper equipment and fertiliser.
"Russia knows how to do this," he wrote. "We have all the opportunities to ensure there is food in other countries, so that there are no crises. Just don't prevent us from working."
Russia's UN Ambassador, Vassily Nebenzia, dismissed as "absolutely false" any suggestion that Russia was to blame for a global food crisis that had been brewing for several years.
He accused Ukraine of holding foreign vessels in its ports and mining the waters.
"The decision to weaponise food is Moscow's and Moscow's alone," Blinken said.
"Some 20 million tonnes of grain sit unused in Ukrainian silos as global food supply dwindle (and) prices skyrocket."
UN Secretary-General Antonio Guterres is trying to broker a deal allowing Ukraine to resume food exports and revive Russian food and fertiliser production to world markets.