Speaking at a G20 foreign ministers' meeting in Indonesia, Russia's Sergey Lavrov said Moscow was “ready for negotiations” to shepherd grain vessels out of Ukrainian ports and towards the Mediterranean.
Turkey and the UN have sought to mediate as Russia and Ukraine blame each other for the evolving global food crisis, caused by a standstill in grain and fertiliser exports that has led to soaring food prices and fears of widespread hunger in Africa.
Ukraine does not trust Moscow's assurances that its navy would pass up the chance to exploit a gap in Ukrainian coastal defences if a shipping corridor is opened.
But there is a wariness among western countries about sending their own warships to keep the peace in what could be seen as an escalation, and Turkey has said its navy could act as a potential escort.
“Ukraine must unblock its ports, clear them or ensure safe passage through minefields, and outside the territorial sea of Ukraine, Russia and Turkey are ready ensure the safety of the relevant ships,” Mr Lavrov said, according to Russian news agency Interfax.
“When the negotiations will continue, I can't say, I haven't been to Moscow for several days, but I can confirm that we are always ready.”
Opening the Black Sea is regarded as essential to shifting the bulk of the 20 million or so tonnes of grain that Ukraine has been unable to export, because workarounds such as rail wagons and river barges have far less capacity.
A spokesman for UN Secretary General Antonio Guterres said on Wednesday that his efforts for a grain deal were continuing but would not comment on potential sticking points in the talks. An adviser to Turkish President Recep Tayyip Erdogan said he was hopeful of a deal and that Russian diplomats were engaging in the talks.
The delicate diplomacy suffered a setback on Thursday when Ukraine publicly criticised Turkey and summoned its ambassador over the release of a Russian-flagged vessel suspected of handling stolen grain.
The cargo ship Zhibek Zholy had been detained by Turkish authorities at the port of Karasu while they investigated what Ukraine said was a “particularly egregious violation” during an emerging food crisis.
But ship trackers showed it apparently heading back towards Russia, and Ukraine said the vessel had been allowed to leave despite “criminal evidence” it had presented to the Turkish authorities.
“Ignoring the appeal of the Ukrainian side, in the evening of July 6, the ship was released by Turkish authorities,” the foreign ministry in Kyiv said.
The Turkish ambassador in Ukraine was summoned to the ministry to “clarify this unacceptable situation,” spokesman Oleg Nikolenko said. There was no immediate response from Ankara.
Turkey has criticised the Russian invasion of Ukraine but sought to maintain cordial relations with Moscow. It hosted tentative peace talks between the two warring parties in March.