Ukrainian President Volodymyr Zelensky has accused Russia of "barbarism" after a missile attack on the port of Odesa.
A landmark deal to resume Ukraine's grain exports - signed hours before the strike - now hangs in the balance.
Kyiv has said preparations are still under way to resume the grain exports despite the apparent breach.
Russia has said it carried out the strike but said they were targeting a Ukrainian naval vessel.
Ukrainian military officials have admitted that a ship was hit but have not said whether it was a military or civilian vessel.
In his comments, President Zelensky said the strike showed that Moscow cannot be trusted to stick to the deal.
The BBC's Paul Adams says Moscow presumably argues - but has not so far said - that the attack did not represent a violation of the agreement signed in Istanbul on Friday.
The text says parties have undertaken not to attack merchant and other civilian vessels and port facilities engaged in the transportation of grain.
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Ukraine is a major grain exporter, but because of the war, about 20m tonnes of grain is trapped in its ports, unable to leave because of Russian forces. This has lead to food shortages and price rises across Africa, which usually relies on Ukraine and Russia for wheat.
Friday's deal, hailed as a "beacon of hope" by the UN, saw the two sides agree to resume grain exports.
The agreement - which took two months to reach - is set to last for 120 days, with a co-ordination and monitoring centre to be established in Istanbul, staffed by UN, Turkish, Russian and Ukrainian officials. It can be renewed if both parties agree.
Just hours after the agreement was signed, two missiles hit Odesa port, according to the Ukrainian military's southern command centre.
Another two missiles were shot down by air defence systems, it added.
The strike caused no significant damage to the port, the command centre said.
The attack has been widely condemned. US Secretary of State Antony Blinken accused Russia of worsening a global food crisis, and said the attack cast "serious doubt" on the credibility of Russia's commitment to the pact.
"Russia must stop its aggression and fully implement the grain deal to which it has agreed," he said on Saturday.