Turkey and the UN are hoping to restart grain traffic after Russia pulled out of a shipping deal in July, complaining that western sanctions were thwarting sales of its own agricultural produce.
Russia withdrew safety guarantees from ships sailing through the Black Sea – although Ukraine said on Friday that two vessels were sailing through a temporary corridor.
The bulk carriers Anna-Theresa and Ocean Courtesy left the port of Pivdennyi on a route hugging the Romanian and Bulgarian coasts, said Ukraine's Infrastructure Minister Oleksandr Kubrakov.
But concerns are high that the Black Sea blockage will worsen a food crisis in countries that depend on Ukrainian grain, many of them in Africa.
UN Secretary General Antonio Guterres said on Thursday he had sent Russia "concrete proposals" to revive the deal.
"We cannot have a Black Sea initiative that moves from crisis to crisis, from suspension to suspension. We need to have something that works and that works to the benefit of everybody," he said.
Turkey's Foreign Minister Hakan Fidan said on a visit to Moscow that reviving the deal to ship Ukrainian grain across the Black Sea was critical for food security.
As Moscow looks for friends in Africa, Russian Foreign Minister Sergey Lavrov said on Friday that "practical work" had begun on donating grain to six "most needy" countries – Burkina Faso, Zimbabwe, Mali, Somalia, the Central African Republic and Eritrea.
The export is "now being organised" and Russia "will also pay for the overhead costs of delivering this free cargo", Mr Lavrov said, as reported by Tass, the pro-Kremlin news agency.
The July 2022 deal allowed ships to pass through the Black Sea on condition that Ukraine and Russia could both inspect the cargo. A joint co-ordination centre was set up in Istanbul. But Russia says the side of the bargain covering its own produce has not been upheld.
When the Black Sea has been blocked, Ukraine has exported grain via rail and river barge, but these routes cannot handle the same volume as ports such as Odesa. The EU said on Thursday it was considering a €600 million ($652 million) proposal to subsidise exports via the Baltic Sea.
Western countries say they have no sanctions on food and blame Russia for turbulence in global food markets.
Turkey wants the warring sides in Ukraine to return to the Black Sea agreement and use it as a basis for broader peace talks.