Ukrainian President Volodymyr Zelenskyy claimed that Saturday's strikes on Odessa – one of three designated export hubs under the recent deal – showed Moscow could not keep its promises.
Russia has said that its missile barrage on a Ukrainian port central to a freshly-inked grain export deal had destroyed Western-supplied weapons, after the attack sparked an outcry from Ukraine's allies.
Saturday's strikes had destroyed a Ukrainian military vessel and arms delivered by Washington, Russia said on Sunday.
"High-precision, long-range missiles launched from the sea destroyed a docked Ukrainian warship and a stockpile of anti-ship missiles delivered by the United States to the Kyiv regime," it said.
"A Ukrainian army repair and upgrade plant has also been put out of order."
Ukrainian President Volodymyr Zelenskyy denounced the strike on the Odessa port – which came just one day after the warring sides struck a deal to resume exports from the facility – as "Russian barbarism".
Zelenskyy said the strikes on Odessa showed Moscow could not be trusted to keep its promises and that dialogue with Moscow was becoming increasingly untenable.
"This apparent Russian barbarism brings us even closer to obtaining the very weapons we need for our victory," Zelenskyy said in a late-night address to the nation.
Under the deal brokered by Turkish President Recep Tayyip Erdogan and backed by United Nations, Odessa is one of three designated export hubs.
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Russia criticised for attacks
Ukrainian officials said grain was being stored in the port at the time of the strike, although the food stocks did not appear to have been hit.
Guterres – who presided over the signing ceremony Friday – "unequivocally" condemned the attack.
The United States "strongly condemned" the attack, with Secretary of State Antony Blinken saying it "casts serious doubt on the credibility of Russia's commitment to yesterday's deal".
Until Sunday, there was no response from Moscow, but Turkish Defence Minister Hulusi Akar said Russia had denied carrying out the attack.
"The Russians told us that they had absolutely nothing to do with this attack," he told Anadolu Agency.
Odessa region officials said the strikes left people wounded and damaged port infrastructure in Odessa, without specifying the number or severity of the injuries.
The first major accord between the countries since Russia's February attack on Ukraine aims to ease the "acute hunger" the UN says an additional 47 million people are facing because of the war.
Ukraine, at the signing, warned it would conduct "an immediate military response" should Russia violate the accord and attack its ships or stage an incursion around its ports.
Zelenskyy said responsibility for enforcing the deal fell to the UN, which along with Türkiye is a co-guarantor of the agreement. Türkiye said it was committed to the accord after the attack.
The deal includes points on running Ukrainian grain ships along safe corridors that avoid known mines in the Black Sea.
Huge quantities of wheat and other grain have been blocked in Ukrainian ports by Russian warships and the mines Kiev laid to avert a feared amphibious assault.