U.N. Secretary-General Antonio Guterres said on Saturday the United Nations is working with the United States and European Union to overcome obstacles to Russian food and fertilisers reaching world markets.

Guterres said that under a U.N.-brokered deal agreed in Turkey last month to resume Ukraine's grain exports cut off since Russia's invasion in February, more than 650,000 metric tonnes 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 fertiliser, which are not subject to sanctions," Guterres said in Istanbul, where he visited a coordination centre overseeing the exports.

He said those countries that imposed sanctions on Russia over its invasion of Ukraine had made clear that the measures did not apply to food and fertilisers, but added there had nevertheless been a "chilling effect" on exports.

"There are a certain number of obstacles and difficulties that need to be overcome in relation to shipping... to insurance and... finance," Guterres told a news conference alongside Turkey's Defence Minister Hulusi Akar.

Guterres said the United Nations was working with Washington and the European Union to remove those obstacles.

"Getting more food and fertiliser out of Ukraine and Russia is crucial to further calm commodity markets and lower prices for consumers," he said.

Russia and Ukraine accounted for around a third of global wheat exports before Russia's Feb. 24 invasion, which Moscow calls a special military operation. Russia is also a major exporter of fertiliser.