President Vladimir Putin has claimed that Russia and the developing world have been "cheated" by a UN-brokered Ukrainian grain export deal.
He vowed to revise its terms and limit the countries that can receive shipments.
But a Ukrainian presidential adviser dismissed the remarks as "flabbergasting".
"The agreements signed in Istanbul concern only one issue, and that is the transfer of cargo ships through the Black Sea," Mykhailo Podolyak, an adviser to Ukrainian President Volodymyr Zelenskiy, told Reuters.
"Russia can't dictate where Ukraine should send its grain, and Ukraine doesn't dictate the same to Russia," he added.
'A scam'
Taking aim at the deal, which was brokered by Turkey and the United Nations, Mr Putin claimed that grain exports are not going to the world's poorest countries as originally intended.
"What we see is a brazen deception, a deception by the international community of our partners in Africa, and other countries that are in dire need of food. It's just a scam," Mr Putin claimed.
In his strongest comments on the topic since the deal was reached in July, Mr Putin warned of a global food crisis if the situation was not addressed.
He vowed to contact Turkey's President Tayyip Erdogan to discuss amending the deal to restrict which countries can receive shipments.
"It is obvious that with this approach, the scale of food problems in the world will only increase ... which can lead to an unprecedented humanitarian catastrophe," he said.
Mr Putin said, without citing a source, that only two of 87 ships, carrying 60,000 tonnes of products, went to poor countries, as he accused the West of acting as colonial states.
"Almost all the grain exported from Ukraine is sent not to the poorest developing countries, but to European Union countries," Mr Putin told an economic forum in the eastern city of Vladivostok today.
UN data shows that Turkey, which is not part of the EU, has been the most frequent single destination for shipments from Ukraine, with cargoes going to China, India, Egypt, Yemen, Somalia and Djibouti as well.
Mr Putin said that Russia would carry on with the deal in the hope that its aims would still be achieved.
Ukraine's agriculture minister said today it was not aware of any formal steps taken by Russia to amend the terms of the deal, which remains the only significant diplomatic breakthrough in the six-month conflict.
Mr Putin also said some restrictions on Russia's fertiliser exports had been eased, but "clever sanctions" were still complicating Russian trade.
The UN said that it had held talks with Russian officials in Geneva to address Mr Putin's concerns over exports of grain and fertilizer.
A spokesman for the agency said that the talks had been positive, constructive and professional.
The grain deal, signed in July, aimed to avert a global food crisis by guaranteeing the safe passage of ships in and out of Ukrainian ports, allowing them to export tens of millions of tonnes of grain.
'What nonsense!'
Mr Putin also denied that he is using its energy exports as a "weapon" after Russia halted natural gas deliveries via a key pipeline to Europe.
The Nord Stream pipeline has been closed for several days after Russia said it would be under repair for an indefinite period of time, reinforcing fears of an energy price crisis in Europe with winter approaching.
Europe, which is heavily dependent on Russian supplies, has accused Moscow of using energy as blackmail in response to sanctions over Russia's military campaign in Ukraine.
"They say that Russia uses energy as a weapon. More nonsense! What weapon do we use? We supply as much as required according to requests" from importers, Mr Putin told the Eastern Economic Forum.
