Wheat prices have risen sharply on global markets after Russia said it would treat ships heading for Ukrainian ports as potential military targets.
Moscow pulled out of a UN deal on Monday that ensured safe passage for grain shipments crossing the Black Sea.
For the past three nights Russia has bombarded Ukraine’s grain facilities in Odesa and other cities.
Moscow also warned that from Thursday any ships going there would be seen as siding with “the Kyiv regime”.
White House spokesman Adam Hodge suggested Russia was planning to hit civilian ships and blame Ukraine.
Russia had laid more sea mines in the approaches to Ukrainian ports, he said, as part of a co-ordinated Russian effort to justify attacking civilian ships.
The Kremlin did not immediately respond to the allegation.
Wheat prices on the European stock exchange soared by 8.2% on Wednesday from the previous day, to €253.75 (£220; $284) per tonne, while corn prices were up 5.4%.
US wheat futures jumped 8.5% – their highest daily rise since just after Russia’s February 2022 invasion of Ukraine.
Earlier President Vladimir Putin said he would return to the international grain agreement immediately if his demands were met. They include lifting sanctions on sales of Russian grain and fertiliser and reconnecting Russia’s agricultural bank to a global payment system.
Russian air strikes on the Black Sea coastal cities meanwhile continued for a third night, leaving more than 20 people wounded in Odesa and Mykolaiv.
Mykolaiv regional governor Vitaliy Kim said 19 people had been hurt in the regional capital, including children. Apartment blocks were targeted and in one building the second and top floor were partially destroyed.
Several people were also wounded in Odesa, when a four-storey building was badly damaged.
Russian-occupied Crimea was also hit overnight, according to its Russian-appointee leader Sergei Aksyonov.
A teenage girl was killed when a drone hit four administrative buildings in the north-west of the peninsula.
Crimea has been hit on three consecutive days. A suspected Ukrainian drone attack from the sea damaged a bridge from the occupied peninsula to southern Russia on Monday. Part of a key motorway was also shut on Tuesday because of explosions at a nearby munitions depot.