Russia is restarting its cooperation in a scheme to export grain from Black Sea ports, days after saying it was withdrawing.

In July Moscow agreed to allow ships to export millions of tonnes of grain and other foodstuffs from Ukraine through a safe corridor in the Black Sea.

After Russia invaded Ukraine, its navy imposed a blockade on Ukraine's Black Sea ports, trapping about 20 million tonnes of grain meant for export inside the country, along with other foodstuffs such as maize and sunflower oil.

Since Ukraine is one of the world's largest exporters of food, this caused food prices to increase worldwide.

However, the two sides signed the Black Sea initiative on 22 July, creating a safe corridor for shipping.

The first ship sailed from Ukraine on 1 August, and up until 28 October, more than 9.3 million tonnes of food had been exported through the Black Sea, according to the UN.

In September, Ukraine exported four million tonnes of food through the Black Sea.

Before Russia's invasion, it had been shipping five million tonnes every month.

Exports have not returned to pre-war levels partly because many companies refuse to send their cargo ships into the corridor.

"They're still afraid of their ships hitting a mine or getting attacked by the Russians," says David Osler, of the shipping journal Lloyd's List.

Some food has gone directly to the poorest countries in the world, and some has been shipped to countries where people are at risk of starvation, under UN humanitarian relief programmes.

However, UN figures show that the bulk of Ukrainian food exported in the last three months has been going to Spain, Turkey, Italy, China and the Netherlands.

Before the war, the top importers of Ukrainian wheat were Egypt, Indonesia and Bangladesh.

The UN said in September that just under 30% had gone to lower-income countries, while 44% had been shipped to high-income countries.

However, the resumption of Ukrainian food exports across the world helped to lower international food prices between July and the end of October, judging by the UN food price index.

Russia and Ukraine both signed deals with Turkey to establish a corridor from Odesa, and two neighbouring ports, to the Istanbul strait.

This corridor is 310 nautical miles long and three nautical miles wide.

Ukrainian vessels guide grain ships in and out of port past mines which Ukrainian naval forces have laid.

Turkish personnel inspect the ships for weapons, at the request of the Russians.

The agreement is due to expire after 120 days, in mid-November, but there are hopes it will be renewed.

Russia said it was temporarily suspending the Black Sea corridor deal after Ukraine attacked its naval base at Sevastopol with drones.

It says a ship using the corridor was involved in the attack, although the UN says there were no ships in the corridor at the time.

Cargo ships continued to sail from Ukraine ports after Ukraine, Turkey and the UN told shipping companies they would still be safe to do so.

However, the global price of wheat on the world's commodity exchanges jumped by over 5%, and maize prices jumped by 2%, on the first day of trading on food exchanges after Russia made its announcement.

Now, Russia has said it is rejoining the agreement, having received assurances from Ukraine that it would not use the corridor to attack Russian forces.

The prices of wheat and other foodstuffs fell back on global markets as a result.

When Russia blockaded Ukraine's ports following it invasion in February, Ukraine tried to export as much produce as it could by land, using lorries and trains.

The EU set up what it called "solidarity lanes," so that Ukrainian grain could be shipped from ports on the Baltic Sea, and also from the Romanian port of Constanta.

However, a lack of road and rail capacity means that Ukraine cannot export more than about 10% of its grain by land