Some 25 ships have carried over 600,000 tons of grain since a deal was brokered between Ukraine and Russia in late July.
UN Secretary General António Guterres said Ukrainian grain shipments offer "hope" for the world during his visit to Turkey on Saturday.
Some 27 ships have carried some 650,000 tonnes of grain and other food from the Black Sea's ports since a deal was brokered between Ukraine and Russia in late July.
Grain exports had been blocked at Ukraine's southern ports amid fighting in the country.
Speaking in Istanbul, Guterres said the shipments were the "beginning of a much longer process".
"More than 650,000 metric tons of grain and other food are already on their way to markets around the world," he said.
“I was so moved watching the wheat fill up the hold of the ship. It was the loading of hope for so many around the world.”
Guterres also inspected a Ukrainian grain shipment aboard the World Food Programme (WFP) ship the Brave Commander.
Guterres said "This ship is going to load in Chornomorsk more than 50,000 tonnes of grain in Ukraine, the largest cargo ever coming out of Ukraine since the war has started."
Guterres also visited the Joint Coordination Center, established to administer the export of agricultural products across the Black Sea.
On Friday, Guterres visited Odesa - Ukraine's main port on the Black Sea - where he called for financial aid for poorer countries facing food shortages.
“As these ports open, I appeal for wealthier countries to also open their wallets and their hearts. After all, the movement of grains doesn’t mean much to countries that cannot afford it,” he said.
“It is time for massive and generous support so developing countries can purchase the food from this and other ports – and people can buy it.
"Developing countries need access to financing now. They need debt relief now. They need resources to invest in their people now."