The Turkish presidency says that a signing ceremony will be held on Friday to unveil a deal brokered between Ukraine and Russia aiming to allow millions of tons of Ukrainian grain to be exported, alleviating a global food shortage.

There was no official confirmation from Moscow or Kyiv that an accord had been reached — only that negotiations would continue on Friday. After officials in both capitals confirmed on Thursday that the two sides were getting closer to an agreement, the Kremlin said on Friday that Russia’s defense minister, Sergei K. Shoigu, had traveled to Turkey.

“We can confirm that the signing is being prepared today,” the Kremlin’s spokesman, Dmitri S. Peskov, told reporters. “But let’s wait — let’s not get ahead of ourselves.”

More than 20 million tons of Ukrainian grain has been trapped in Ukraine’s Black Sea ports since Russia’s full-scale invasion began in February, cutting off grain exports from one of the world’s breadbaskets, exacerbating famine in Africa, and undermining international food supply chains already battered by the pandemic.

In a war in which President Vladimir V. Putin has shown a willingness to target civilians and weaponize energy and food, Russia’s blockage of Ukraine’s grain exports has rippled across the world and has been among the gravest international repercussions of Moscow’s attack on its neighbor.

Russia’s de facto blockade of the Black Sea caused Ukraine’s exports to drop to one-sixth of their prewar level. Officials have sought to break the impasse for months, as international aid organizations have made increasingly dire forecasts about an increase in food prices and rising rates of hunger and starvation. At the same time, Western countries have treaded carefully, mindful of the risks of ensnaring NATO in the war.

Andrei Rudenko, Russia’s deputy foreign minister, told Interfax on Thursday that the negotiations are proceeding “quite dynamically, and the parties are participating in them constructively.”

He added, “Therefore, we hope we’ll be able to reach a consensus in the near future.”

Any prospective agreement could fall apart at the 11th hour. And if a deal goes through, there could be major obstacles to implementing an agreement between the two warring nations. It is also unclear how much grain would be released and what condition it would be in after being stored for months in a war zone.

The United Nations said on Thursday that its secretary general, António Guterres, had landed in Istanbul as part of his effort “to ensure full global access to Ukraine’s food product and Russian food fertilizer.”

“The situation remains a little bit fluid, so I can’t really say when something will be signed,” said the United Nations deputy spokesman, Farhan Haq, earlier on Thursday. “But as you can see from the fact that he is traveling to Istanbul, we are moving ahead with this.”

Last week, after meeting in Istanbul with negotiators from Ukraine, Russia and Turkey, Mr. Guterres told reporters that a deal was “technically done” and that he would interrupt his vacation and travel to Istanbul for the signing of it.

Until now, one of the major hurdles to an agreement were the mines Ukraine had placed in its ports on the Black Sea Coast to deter Russia’s warships. In late June, Mr. Guterres outlined the primary elements of a deal proposed by the United Nations and Turkey that would solve that problem.

According to three senior government officials, Mr. Guterres said Ukrainians had agreed to remove only a few of the mines and have their own Navy or Coast Guard captains steer freighters to international waters. Foreign crews would then take the ships to Istanbul, before continuing to other destinations.

A control center would be set up in Istanbul to oversee the operation, and Turkish officials would play the main role in checking the vessels to guarantee to Russia that the empty ships were not ferrying weapons back to Ukraine.

Two top senior European officials with direct knowledge of the talks, who could not be identified because of the sensitivity of the negotiations, also said on Thursday that they were optimistic a deal would be struck on Friday in Istanbul. The European Union is not party to the negotiations.

For its part, the Russian side has insisted its own grains and fertilizers are stuck because of E.U. measures — an allegation the Europeans have vehemently rejected as propaganda. Russian grains and fertilizers are not sanctioned by the European Union.

On Thursday, the bloc published a set of legal clarifications that sought to dispel any doubt that companies can export Russian grains and fertilizers without running afoul of sanctions.

Ned Price, a U.S. State Department spokesman, said the Biden administration would welcome an agreement, but stressed the importance of ensuring that Russia abides by and implements it.

Ivan Nechepurenko contributed reporting from Tbilisi, Georgia and Edward Wong from Lewes, Del.