A string of explosions rocked Ukraine’s southern city of Odesa on Saturday, hitting one of the country’s most important ports less than 24 hours after a deal was signed to secure the transit of millions of tonnes of grain through Black Sea routes.

The strikes risk undermining the deal to facilitate the shipping of Ukrainian grain, brokered by the United Nations and Turkey, before the agreement could even be put into action. The deal was seen as critical for shoring up global supplies after a steep drop in Ukrainian grain exports raised fears of food shortages in poorer nations.

Ukraine’s southern military command said on Saturday that Russian forces had fired four Kalibr cruise missiles at Odesa, Ukraine’s largest port. “Two rockets were shot down by air defence forces, two hit port infrastructure facilities,” it wrote in a statement posted on its Facebook page.

Ukrainian President Volodymyr Zelensky said the strike showed Moscow could not be trusted to implement the deal. However, public broadcaster Suspilne quoted the Ukrainian military as saying the missiles had not caused significant damage and a government minister said preparations continued to restart grain exports from the country’s Black Sea ports.

Officials said it was the first time since the war began that the port in Odesa had been targeted.

Turkey’s defence minister said Russian officials had told Ankara that Moscow had “nothing to do” with the strikes on the port. The ministry did not reply to a Reuters request for comment.

The condemnation from Ukraine was swift. Oleg Nikolenko, the spokesman for the country’s foreign ministry, said on Facebook that with the strikes, President Vladimir V. Putin of Russia had “spit in the face” of the United Nations secretary-general, António Guterres, and President Recep Tayyip Erdogan of Turkey after the two “expended enormous effort to reach this agreement”.

The United Nations secretary-general’s deputy spokesman denounced the strikes, saying in a statement that full implementation of the agreement was “imperative”.

The blast wave from the missiles hitting the port could be felt from miles away, though it was unclear precisely where they struck. The huge port sprawls for miles along Odesa’s Black Sea coast with towering silver grain silos clustered in several different places.

Russia may not have technically violated the deal, since it did not pledge to avoid attacking the parts of the Ukrainian ports that are not directly used for the grain exports, according to a senior UN official. If there were military targets nearby, Russia may have been trying to exploit a loophole.

Still, the damage appeared to be extensive, and Mykola Solskyi, the country’s agriculture minister, said the strikes would affect Ukraine’s efforts to export grain.

“If you attack a port, you attack everything,” he said in a telephone interview. “You use a lot of the same infrastructure for oil, for grain. It has an impact on everything — it doesn’t matter what you hit.”

Solskyi added that some of the infrastructure destroyed was “important for processing all imports”, but said that Ukraine would proceed as if the grain deal would still go into effect.

“We understand that we still have a war with Russia,” he said. “Our agreement was with the United Nations and Turkey, not with Russia.”

Infrastructure minister Oleksandr Kubrakov said that “we continue technical preparations for the launch of exports of agricultural products from our ports”.