Apart from being major global wheat suppliers, Russia is also a large fertiliser exporter and Ukraine a significant producer of corn and sunflower oil.
A deal is seen as vital for food security, notably among developing nations, and for stabilising markets.
But Guterres warned there was still “a long way to go” before there would be peace talks to end the war.
Several Ukrainian cities have reported heavy Russian shelling and Ukrainian Foreign Minister Dmytro Kuleba was earlier downbeat on prospects for peace.
Ukrainian officials said there had been sustained Russian shelling across Donetsk province, which Moscow aims to capture to complete its seizure of the industrialised Donbas region of eastern Ukraine.
Russian media reported Ukrainian armed forces launched a fresh missile attack in a strategically important Russian-held southern area of Kherson that Kyiv is hoping to retake.
RIA news agency quoted the Russian-backed administration of Kherson region as saying Russian air defences shot down five missiles fired at the town of Nova Kakhovka, while the debris of two of the missiles fell near a factory.
“According to preliminary information, there’s been another hit on a Russian munitions plant, at Sokol,” Serhiy Khlan, an advisor to the Ukrainian head of Kherson province, wrote on Facebook.
A separate strike on Tuesday in Nova Kakhovka killed 52 people, Ukraine’s military has said. The Moscow-appointed deputy head of Kherson region said that two people were killed, seven were missing and 90 were injured.
On Wednesday TASS quoted a separatist official, Vitaly Kiselyov, as saying Russian and proxy forces had entered the town of Siversk in Donetsk province and could take it in a couple of days.
Russia had not conducted any new assaults on the frontline that includes Siversk, but that the town had been fired on by artillery, Ukraine’s armed forces said.
Reuters could not independently verify the battlefield accounts.
Russia’s February 24 invasion of Ukraine caused Europe’s biggest conflict since 1945. Millions have fled and thousands have been killed while cities have been reduced to rubble and fears of a wider conflict in the West have grown.
The Kremlin says it is engaged in a “special military operation” to demilitarise and “denazify” Ukraine. Both Kyiv and Western nations say that is a pretext for an unprovoked war of aggression.
Ukraine and the West have accused Russia of exacerbating a global food crisis and fuelling inflation.
Moscow has blamed Ukraine, saying it refuses to remove mines that it scattered around its coastline to protect itself from Russia’s attack and that threaten shipping.
The Kremlin also says Western sanctions make it harder for Russia to fund and insure its own maritime freight services.
Russia’s Interfax news agency quoted Pyotr Ilyichev, head of the international organisations department at Russia’s foreign ministry, as saying Russia wanted to control and inspect grain vessels itself to rule out arms smuggling.
Before progress was announced, diplomats said that the plan under discussion included Ukrainian vessels guiding grain ships in and out through mined port waters; Russia agreeing to a truce while shipments move; and Turkey – supported by the United Nations – inspecting ships to allay Russian fears of weapons smuggling.
RIA quoted an unnamed diplomatic source as saying Russia’s demands included the removal of “obstacles to exports” created by sanctions, citing the areas “of shipping insurance, logistics, transportation services and banking operations”.
A senior UN official, speaking on condition of anonymity, said most of the sticking points in the talks to resume Ukraine Black Sea exports had been overcome, describing the discussions in Istanbul as a “breakthrough”.
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