Grain silos in Ukraine's government-controlled territory are about half full in the run-up to this year's harvest, meaning crops could be left in the ground if Russia continues its port blockade, the head of the Ukraine Grain Association said on Wednesday.

Russia's Feb. 24 invasion of Ukraine, the world's fourth largest grains exporter, halted Kyiv's Black Sea grain exports, threatening a global food crisis and triggering protests across developing countries.

There are some 30 million tonnes of grain stored in Ukrainian-held territory out of capacity of around 55 million tonnes, Mykola Gorbachov told an International Grains Council (IGC) conference in London, adding there were 13-15 million tonnes of storage capacity in Russian-occupied areas.

Without access to its Black Sea ports, Ukraine would at best be able export 20 million tonnes of grains next year, Gorbachov said, adding these would likely be more sunflower seeds than corn and wheat.

Ukraine, which shipped 44.7 million tonnes of grains in 2020/21, is currently trying to export its crop via road, river and rail but logistics difficulties limit volumes to a maximum of around two million tonnes a month using these routes.

"I can tell you we won't find a solution (for) exports (without Black Sea port access)," Gorbachov told the IGC conference.

"The harvest will start in one month. We have enough storage capacity for wheat and barley. The problem will start in November when farmers start to dry corn."

Gorbachov estimated half of Ukraine's corn crop would be left in the ground, unharvested, if Russia maintains its blockade of the Black Sea ports. Ukraine is the world's fourth-largest corn exporter and largest sunflower oil exporter.

Turkish efforts to negotiate safe passage for grain stuck in Black Sea ports met resistance on Wednesday as Ukraine said Russia was imposing unreasonable conditions and the Kremlin said free shipment depended on an end to sanctions.

Gorbachov said the Turkey-Russia talks were not likely to lead to the security guarantees required to ship Ukraine's grains and instead called on the United States, Britain and France to provide safe convoy for vessels carrying Ukraine's crop.

"I don't believe any company will load (Ukrainian) grain if the Russians sign something with the Turks. Insurance won't cover this risk. We need international guarantees, only in this case the grain will move," he said.

Ukraine and the West accuse Moscow of weaponising food supplies. Moscow blames the situation on what it says are Ukrainian mines in Black Sea waters and on international sanctions against Russia, the world's largest wheat exporter.

Around 22 million tonnes of grain were stuck in Ukraine as of early May. As prices surge, United Nations agencies are having to cut food rations for refugees by up to half in poor parts of the world like the Sahel.