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 deal this week that had guaranteed safe passage for grain shipments through the Black Sea.
A White House spokesperson accused Russia of planning to blame Ukraine for attacks on civilian ships.
Russia’s President Vladimir Putin said he would return to the grain agreement immediately if his demands were met.
They include reconnecting Russia’s agricultural bank to a global payment system.
A Russian air strike on the Ukrainian port city of Mykolaiv wounded 18 people on Wednesday night, according to a local official.
The region’s governor Vitaliy Kim said nine of the injured, including five children, were taken to hospital for treatment.
Other air strikes were reported on the port of Odesa.
