Ukraine's defence ministry said a grain silo was damaged in the Danube port of Izmail in the Odesa region.
"Ukrainian grain has the potential to feed millions of people worldwide," the ministry wrote on messaging platform X.
The port, across the river from NATO-member Romania, has served as the main alternative route out of Ukraine for grain exports since Russia reintroduced its de facto blockade of Ukraine's Black Sea ports in mid-July.
Video released by the Ukrainian authorities showed firefighters on ladders battling a huge blaze several stories high in a building covered with broken windows.
Several other large buildings were in ruins, and grain spilled out of at least two wrecked silos.
There were no reports of casualties, Odesa region governor Oleh Kiper wrote in a post on the Telegram messaging app.
"Unfortunately, there are damages," President Volodymyr Zelenskiy said on Telegram.
"The most significant ones are in the south of the country.
"Russian terrorists have once again attacked ports, grain, global food security."
An industry source also confirmed Izmail was the main target of the attack, describing the level of damage there as "serious".
Chicago wheat prices jumped four per cent following Wednesday's attack and were still up about 2.5 per cent later in the morning, with traders worried anew about a hit to global supplies from driving Ukraine, one of the world's top exporters, off the market.
Russia has relentlessly attacked Ukrainian agricultural and port infrastructure for more than two weeks since refusing to extend an agreement that had lifted its war-time blockade of Ukrainian ports last year.
The Kremlin said President Vladimir Putin had spoken by telephone to the grain export deal's sponsor, Turkish President Tayyip Erdogan.
Putin reiterated Russia's condition for rejoining the grain deal: that a parallel deal improving terms for its own food and fertiliser exports be implemented.
Those exports are already exempt from sanctions, which the West says Moscow aims to undermine by applying pressure to the global food supply.
Moscow has described its recent attacks as retaliation for a Ukrainian strike on a bridge to Crimea, used to supply its troops in southern Ukraine.
United States ambassador Bridget Brink listed recent Russian targets.
"Homes. Ports. Grain silos. Historic buildings. Men. Women. Children," she said in a statement released by the embassy.
"Round-the-clock and intensifying Russian strikes on Kryvyi Rih, Kharkiv, Kyiv, Kherson make it clear once again Russia has no desire for peace, no thought for civilian safety, and no regard for people around the world who rely on food from Ukraine."
Kyiv says the goal of the strikes is to reimpose Russia's blockade by persuading shippers and their insurance companies that Ukrainian ports are unsafe to resume exports.
"It is the enemy's priority to convince the international community and shipowners in particular that the ports of Odesa, the ports of Ukraine, are unreliable places to operate, to work, to load, to navigate - to convince them that navigation in the direction of Ukrainian-controlled ports is dangerous," military spokeswoman in southern Ukraine Natalia Humeniuk said.
Ukraine's Danube River ports such as Izmail accounted for about a quarter of grain exports before Russia pulled out of the Black Sea deal, and have since become the main remaining route out, with grain loaded onto barges and shipped to Romania's Black Sea port of Constanta for shipment onwards.
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