Oleksandr Kubrakov, Ukraine’s infrastructure minister, told the Financial Times that “all of our activity won’t cover even 20 per cent of what we could do through the Black Sea ports”.

Ukraine and its western allies are searching for ways to get up to 20mn tonnes of grain out of the country and clear storage space for this year’s harvest. The crisis threatens tens of millions of people in countries across the Middle East and Africa that rely on Ukraine’s cereals.

Trucks face lengthy delays at the border with Poland and Romania, while moving grain by rail is difficult because trains in the EU and Ukraine run on different gauges of track. Russia has repeatedly bombed the alternative routes, including those leading to Romania by road or rail, where grain is then loaded on barges that sail down the Danube and into the Black Sea.

The EU has simplified its procedures and Ukraine is offering additional guarantees to European barges and trucks after most western insurers shied away because of the risk.

Despite those efforts, Kubrakov said it would prove insufficient. “Everyone is doing superhuman activity, and the [amount exported] is growing every month . . . in the short term it could go up to 30 per cent [of Ukraine’s Black Sea exporting capacity],” he said.

Russia has captured much of Ukraine’s breadbasket in the south and is making progress in the eastern Donbas industrial region, the scene of the fiercest fighting three months into Putin’s invasion of Ukraine.

Since late February, Russia has seized between 400,000 and 500,000 tonnes of grain from occupied territories. Some Ukrainian farms have been hit by air strikes and artillery fire.

Russian president Vladimir Putin has blamed the food crisis on sanctions against Russian exports and said Moscow would only lift the blockade if the restrictions are lifted. On Friday he said in a state television interview: “The problem of exporting grain from Ukraine does not exist.”

Kubrakov warned that Russia’s actions risked creating famine “on a global scale” and Moscow was acting like “total pirates”.

“They don’t care about the lives of these people in Africa,” he said. “They?re telling them: ‘We don’t care about you. We are only worried about sanctions against us. Now you are hostages. “‘

Kubrakov said converting a single railway line to the EU standard would cost $2bn to $3bn, with more investments needed to expand capacity at border crossings.