UN Secretary-General Antonio Guterres arrives at a boat to sail a ship carrying Ukrainian grain, at Zeyport in Istanbul, Turkey August 20, 2022. (REUTERS/Murad Sezer)

Guterres said that under a UN-brokered deal agreed in Turkey last month to resume Ukraine's grain exports cut off since the conflict between Russia and Ukraine in February, more than 650,000 metric tons of grain and other food was already being exported.

"The other part of this package deal is the unimpeded access to the global markets of Russian food and fertilizer, which are not subject to sanctions," Guterres said in Istanbul, where he visited a coordination center overseeing the exports.

He said that there are a certain number of obstacles and difficulties that need to be overcome in relation to shipping, to insurance, and finance. The UN was working with the US and the EU to remove those obstacles, he added.

Russia and Ukraine accounted for around a third of global wheat exports before their conflict, which Moscow calls a special military operation. Russia is also a major exporter of fertilizer.