The high-stakes meeting involving UN and Turkish officials in Istanbul Wednesday broke up after slightly more than three hours with an agreement to meet again in Turkey next week.
UN Secretary-General Antonio Guterres said the talks had provided a "ray of hope to ease human suffering and alleviate hunger around the world" but cautioned that while he was optimistic, a deal was "not yet fully done".
NATO member Turkey has been using its good relations with both the Kremlin and Kyiv to try to broker an agreement on a safe way to deliver the grain.
Turkey's defense minister signaled that a final agreement could be announced at the next talks.
"At this meeting, which we will hold next week, all the details will be reviewed once again and the work we have done will be signed," Hulusi Akar said in a statement.
Shipments across the Black Sea and out to the world through the Bosphorus have been blocked both by Russian warships and by measures taken by Ukraine to deter an amphibious assault from the water.
The two sides entered the talks saying that a deal was close but some contentious issues remained.
The negotiations are being complicated by growing suspicions that Russia is trying to export grain it has stolen from Ukrainian farmers in regions under its control.
Russian authorities in Ukraine's southern region of Kherson on Wednesday countered with accusations that Kyiv's forces were deliberately burning crops and mining fields.
Russia said Tuesday that its requirements for agreement included the right to "search the ships to avoid the contraband of weapons" -- a demand rejected by Kyiv.
The initial understanding announced by Akar said the two countries agreed on "joint controls" at ports and on ways to "ensure the safety of the transfer routes".
Turkey says it has 20 merchant ships waiting in the region that could be quickly loaded and sent to world markets.
Kyiv has asked that a friendly country such as Turkey accompany its ships along safe "grain corridors" that avoid known locations of Black Sea mines.
Experts say de-mining the Black Sea is a complex operation that could take months -- too long to address the growing global food crisis.