Maritime grain exports from Ukraine were halted on Saturday, when Russia suspended participation in the agreement which had been allowing shipments from Black Sea ports to Turkey. Moscow blames drone attacks on its vessels in Crimea for the suspension.
The deal to free grain exports, signed between Russia and Ukraine in July, was due to be renewed on 19 November.
The agreement has allowed more than nine million tonnes of Ukrainian grain to be exported, directly easing the global food crisis caused by the conflict.
On Saturday, Russia said it was halting its participation after its army accused Kyiv of a "massive" drone attack on its Black Sea fleet, which Ukraine labelled a "false pretext".
US President Joe Biden called the move "outrageous" while Secretary of State Antony Blinken said Moscow was "weaponising food".
On Sunday, the EU urged Russia to "revert its decision."
Ukraine's foreign minister claimed on Twitter that Russia was blocking "two million tons of grain on 176 vessels already at sea" that he said was "enough to feed seven million people."
He accused Moscow of having planned to "resume its hunger games" in advance and said the Black Sea explosions were "220 kilometres away from the grain corridor."
Ukrainian President Volodymyr Zelensky called the Russian move "an absolutely transparent intention of Russia to return the threat of large-scale famine to Africa and Asia".
Stephane Dujarric, spokesman for the UN secretary-general, said: "It is vital that all parties refrain from any action that would imperil the Black Sea Grain Initiative which is a critical humanitarian effort".
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