Russia's withdrawal from the Black Sea grain deal is already having an adverse effect on food prices only a week on, the head of the UN said on Monday.
Moscow quit the deal last Monday and withdrew safety guarantees for shipping, complaining it was not getting its side of the bargain.
Ukraine and Russia are two of the world's top producers of wheat, barley and sunflower oil and the war has been blamed for worsening global hunger. Countries including Lebanon, Tunisia and Somalia are highly reliant on food imports from Ukraine in peacetime.
Mr Guterres spoke of "broken food systems" causing millions to go hungry, in an address to a UN summit in Rome focusing on food and sustainable farming.
He said many communities were "one shock away" from falling into food insecurity or famine.
"The dire picture has grown bleaker with the Russian Federation’s termination of the Black Sea grain initiative," said Mr Guterres.
"We are already seeing the negative effect on global wheat and corn prices, which hurts everyone, but this is especially devastating for vulnerable countries struggling to feed their people. As food prices rise, the hopes of developing countries fall."
Mr Guterres said he "remains committed" to facilitating exports from both Ukraine and Russia, which complains that it cannot sell its own food and fertiliser.
The Kremlin ignored pleas by Mr Guterres to preserve the deal in return for possible sanctions relief on a Russian bank. Moscow's forces have since attacked the Ukrainian port of Odesa, damaging its harbours and storage sites.
"I call on the Russian Federation to return to the implementation of the Black Sea initiative in line with my latest proposal and I urge the global community to stand united for effective solutions in this essential effort," Mr Guterres said.
The summit in Rome will also focus on the effects of climate change on food production, a topic the UAE's presidency of Cop28 has said it will address at the talks in Dubai.
Italian Prime Minister Giorgia Meloni also urged Russia to reconsider its stance on the grain deal. She said the war in Ukraine had aggravated food insecurity in African countries "already tested by long periods of drought" and difficult climate conditions.
As a result some nations in Africa "are now weaker and easier prey for terrorism and fundamentalism", she said.