ISTANBUL - Russia and Ukraine agreed Friday to restart shipments of blockaded grain, in a step toward easing a global crisis that has exposed tens of millions of people, especially in Africa and the Middle East, to the threat of acute hunger, the United Nations secretary general announced.

One of the two agreements signed Friday in Istanbul - which were brokered by the United Nations, aided by Turkey and amounted to a diplomatic victory for both - is intended to guarantee the safe passage of commercial ships from the Ukrainian port of Odessa and two other ports, which are currently cut off by a Russian naval blockade, U.N. officials said before the signing. A parallel agreement is supposed to facilitate Russian grain and fertilizer exports, they said.

The agreements are in force for a period of 120 days and are renewable, officials said.

Before Russia’s invasion of Ukraine in late February, the two countries were among the world’s top producers and exporters of grain, cooking oil and fertilizers. Last year, Ukraine accounted for 10% of global wheat exports, according to the United Nations. More than 20 million tons of grain have been stuck in Ukraine’s Black Sea ports, causing worldwide shortages and fears of worsening hardship to come.

For all the complexity involved in the negotiations, the grain agreement appeared to depend on goodwill that is in short supply, resting in large part on Russian assurances it would not attack merchant ships or port facilities involved in the initiative. Even so, officials expressed optimism.

“Today, there is a beacon on the Black Sea,” United Nations Secretary General António Guterres said at a ceremony announcing the initiative, which was attended by delegations from Ukraine and Russia and Turkish President Recep Tayyip Erdogan.

“A beacon of hope, a beacon of possibility, a beacon of relief, in a world that needs it more than ever.”

“It will bring relief for developing countries on the edge of bankruptcy and the most vulnerable people on the edge of famine,” he added.

The agreements were the fruit of conversations Guterres had with the leaders of Ukraine and Russia in April to solve the spiraling food crisis, a senior U.N. official said in a briefing with reporters Friday. Turkey, which maintains good ties with both countries and controls passage through the Bosporus, the entrance to the Black Sea, took an active mediating role.

For months the discussions stumbled, in a sign of absent trust between the warring parties. Ukrainian diplomats complained that their security concerns were not being acknowledged, as Russia downplayed the scope of the global food crisis. Ship insurance underwriters had to be assured that vessels would not be attacked, struck by mines or face other hazards in an active war zone.

The final agreement over Ukrainian grain shipments rests on a complex regime that establishes safe channels through the Black Sea and inspections to ensure that weapons are not sent to Ukraine through those channels, the U.N. officials said. Despite early speculation, there is to be no large-scale demining of Ukraine’s ports, a process that was considered too time consuming. Ukrainian pilots will guide commercial vessels from the ports. Minesweepers will be used as needed, officials said.

There would be no military escorts of the ships, whose passage will be monitored from a coordination center in Istanbul staffed by representatives of the parties to the agreements.

A parallel agreement is supposed to facilitate the export of grain and fertilizers from Russia, though its utility was unclear. Those commodities are not subject to United States or European Union sanctions. A second U.N. official said they hoped it would help bring down soaring costs of fertilizers that could impact yields for the next harvest. Both U.N. officials spoke on the condition of anonymity because when they spoke the agreements had not yet been signed.

In a sign of the sensitivities weighing on the deal, Mykhailo Podolyak, an adviser to Ukraine’s president, said on Twitter on Friday afternoon that Ukraine was not signing a direct agreement with Russia, but rather with Turkey and the United Nations. Russia would sign a “mirror” agreement, he said.

And there would be “no presence” of Russian representatives in Ukrainian ports he said. “In case of provocations,” he added, “an immediate military response.”