It will be difficult to feed the world if Russia pulls out of the Black Sea grain initiative, the head of the World Food Programme has warned.
Cindy McCain said the deal must be renewed before it expires on May 18.
She told the BBC the WFP had been sourcing grain from other sources to distribute to countries around the world but it had not been able to feed as many people, owing to rising costs.
The conflict “has managed to completely cascade around the world difficulties to be able to feed people”, she said.
Ms McCain said she was “not confident” Russia would agree to an extension of the deal.
“No, I'm not, I'm not confident that they will. The things I've been hearing is that … it's 50-50 right now.
“It worries me very much. And it should worry everybody else too.”
The deal, brokered by the UN and Turkey, has enabled the export of more than 28 million tons of Ukrainian grain during the war.
But Russia has threatened to refuse to extend the agreement, returning to a full-scale maritime blockade of the Black Sea if the West fails to meet its demands, which include allowing Russia’s state agricultural bank to rejoin the international Swift payments system and lifting western sanctions against Russians including fertiliser tycoon Dmitry Mazepin.
Officials from Russia, Ukraine, Turkey and the UN met in Istanbul on Thursday to discuss the future of the deal, but the meeting appeared to end without agreement.
Ms McCain told the BBC it may be hard for Ukrainian farmers to deliver a harvest this year.
“I know that there are some farms that are still operating. But you have to remember, a large majority of the land where the crops were grown before is now mined, with landmines,” she said.
“This is a tragic situation. And if the conflict were to end today, we'd be years being able to clear the land and clear the properties to make sure that it was safe to plant and safe to put livestock on.”
Meanwhile, Russia's Defence Ministry has denied reports that Ukrainian forces had broken through in various places along the front lines in the war.
Moscow reacted after Russian military bloggers, writing on Telegram, reported what they said were Ukrainian advances north and south of the city of Bakhmut, with some suggesting a long-awaited counter-offensive by Kyiv's forces had started.
Ukrainian President Volodymyr Zelenskyy earlier said the offensive had yet to start.
“Statements circulated by individual Telegram channels about 'defence breakthroughs' that took place in different areas along the line of military contact do not correspond to reality,” the Russian Defence Ministry said in a Telegram post.
“The overall situation in the area of the special military operation is under control,” it said, using the Kremlin's description of the war zone in Ukraine.
Moscow acknowledges that it is a “very difficult” military operation.
Ukraine says it has pushed Russian forces back over the past several days near Bakhmut, while a full-blown counter-offensive involving tens of thousands of troops and hundreds of western tanks is still being prepared.
“We still need a bit more time,” Mr Zelenskyy said in an interview with European broadcasters.
Ukrainian military analyst Oleksandr Musiyenko said Kyiv's allies understand a counter-offensive “may not result in the complete eviction of Russian troops and the definitive defeat of Russia in all occupied areas”.
“We have to be ready for the war to continue into next year — or it could end this year,” Mr Musiyenko told Ukrainian NV Radio. “It all depends on how the battles develop. We can't guarantee how the counter-offensive will develop.”
Yevgeny Prigozhin, head of Russia's Wagner private army which has led the fight in Bakhmut, on Thursday said Ukrainian operations were “unfortunately, partially successful”. He called Mr Zelenskyy’s assertion that the counter-offensive had not yet begun “deceptive”.
Mr Zelenskyy said Ukrainian forces had already received enough equipment from western allies for their campaign but were waiting for the full complement of armoured vehicles to arrive.
In a major step up in western military support for Ukraine, Britain said it was sending Storm Shadow cruise missiles that would give Kyiv the ability to strike deep behind Russian lines.
The missiles “are now going into, or are in, the country itself,” Defence Secretary Ben Wallace told Parliament in London. He said the missiles were being supplied so they could be used within Ukraine.
Western countries including the US had previously held back from providing long range weapons for fear of provoking Russian retaliation. Mr Wallace said Britain had weighed the risk.
