Newly declassified U.S. intelligence shows that a Russian naval blockade has halted maritime trade at Ukrainian ports, in what world leaders call a deliberate attack on the global food supply chain that has raised fears of political instability and shortages unless grain and other essential agricultural products are allowed to flow freely from Ukraine.

Russia’s navy now effectively controls all traffic in the northern third of the Black Sea, making it unsafe for commercial shipping, according to a U.S. government document obtained by The Washington Post.

The document, based on recently declassified intelligence, analyzed heat signatures emitted by Russian ships to reveal dense areas of naval activity along portions of Ukraine’s southern coast and the Crimean Peninsula, which Russia occupied and annexed in 2014. The blockade that ensued following Russia’s invasion in February halted civil maritime traffic, “entrapping Ukrainian agricultural exports and jeopardizing global food supplies,” according to a U.S. official, who spoke on the condition of anonymity to describe the intelligence.

“The impact of Russia’s actions cannot be understated as Ukraine’s seaborne exports are vital to global food security,” the official stated. “Ukraine provides about 10% of the world’s wheat exports, and the vast majority of those exports — approximately 95% in 2020 — departed via Black Sea ports.”

In recent days, world leaders have warned that Russia’s blockade poses one of the most dire threats to global stability since the war began. Ukraine is a global food basket. The country is the world’s largest exporter of sunflower oil, the fourth largest exporter of corn, and the fifth largest exporter of wheat.

Speaking at the World Economic Forum in Davos, Switzerland, on Tuesday, European Commission President Ursula von der Leyen said that 20 million tons of wheat are struck in Ukraine. Russia has deliberately bombed Ukrainian grain warehouses and is “hoarding its own food exports as a form of blackmail,” she said, “holding back supplies to increase global prices, or trading wheat in exchange for political support. This is using hunger and grain to wield power.”

Politically fragile countries are especially vulnerable, von der Leyen said, noting that bread prices in Lebanon have increased by 70% and food shipments from Odessa to Somalia have stopped because of Russia’s actions.

Secretary of State Antony Blinken echoed the grim assessment last week in remarks at the United Nations, calling Russia’s blockade “a deliberate effort” to destabilize the world’s food supply.

Since Russia issued a warning to mariners in February that significant areas of the Black Sea were closed to commercial traffic, “the Russian military has repeatedly blocked safe passage to and from Ukraine by closing the Kerch Strait, tightening its control over the Sea of Azov, stationing warships off Ukrainian ports. And Russia has struck Ukrainian ports multiple times,” Blinken said.

“The food supply for millions of Ukrainians — and millions more around the world — has quite literally been held hostage by the Russian military,” he said.

For weeks, Ukrainian President Volodymyr Zelensky has called on Western powers to break the blockade. In his remarks to the Davos forum, Zelensky said Russian forces were blocking Ukraine from exporting 22 million tons of grain, sunflower and other foods, which were “rotting” in Ukraine.

“If we do not export [grain] in the coming months, if there are no political agreements with Russia through intermediaries — there will be famine, there will be a catastrophe, there will be a deficit, there will be a high price,” Zelensky warned.

But Western powers have few options for ending the Russian blockade.

The United States has no vessels in the Black Sea, Gen. Mark A. Milley, the chairman of the Joint Chiefs of Staff, told reporters at a news conference at the Pentagon on Monday. The Biden administration has steadfastly resisted any direct military involvement in the war.

Ukraine has focused on preventing an amphibious assault by Russian forces into key coastal cities and areas, which could help Russia to solidify its control in the east of the country, where Moscow has concentrated its energy after failing to capture the capital, Kyiv, and other key cities in central Ukraine.

“Right now it’s a bit of a stalemate [in the Black Sea region] between the Ukrainians wanting to make sure that there’s not any sort of amphibious landing against Odessa,” a key strategic port, Milley said. The area has become “a no-go-zone for commercial shipping,” he said.

The Ukrainian military has notched some notable successes against the Russian navy, sinking the Black Sea flagship Moskva, with some assistance from intelligence provided by the United States. But those strikes have been insufficient to reopen Ukrainian ports.

The European Commission has proposed exporting Ukraine’s wheat and other grains by rail, road or by river. But it’s unclear how allied forces could protect those shipments from Russian assault.

Denmark plans to send Harpoon anti-ship missiles and a launcher to Ukraine, which could help pierce the blockade and get food exports flowing again. But it may take months to train Ukrainian military personnel on how to use the weapons and integrate them into the country’s coastal defenses, according to military experts.

“Even before Russia’s full-scale invasion of Ukraine, 2022 was forecast to be the most food-insecure year on record globally, making supply from Ukraine even more critical,” Michael Carpenter, the U.S. ambassador to the Organization for Security and Cooperation in Europe, said in remarks last week in Vienna.

Carpenter cited Ukrainian government reports that Russian forces had stolen 400,000 tons of Ukrainian wheat, as well as farm equipment, which was shipped back to Russian territory. He said Russian forces had also destroyed roads, railways and rail stations needed to transport products for export, in addition to its blockade of sea ports.

“As a result of Russia’s aggression, global food shortages are increasing and food prices are rising, exacerbating suffering and hardship for millions of vulnerable people across the globe,” Carpenter said, noting a World Food Program estimate that the war could drive 47 million additional people worldwide “into acute food insecurity.”

Commercial satellite imagery appears to confirm some of Carpenter’s and the Ukrainian government’s allegations. Photos taken last week and published by Maxar Technologies showed Russian ships loading grain at Sevastopol, a port in Russian-occupied Crimea.

Russian officials have denied they are attempting to cut off Ukrainian exports and blamed Western sanctions for disruptions to global food supplies.

“These bans destroyed in a day, long-term, convenient, comfortable transport and logistics chains,” Russian Foreign Minister Sergei Lavrov said in remarks in Oman earlier this month.

Offering no evidence to support his claims, Lavrov accused Ukrainian authorities of refusing to let “dozens of vessels leave their ports, including those that deliver wheat to various parts of the world. They have even closed their ports by mining the exit ways, making it impossible for these vessels to leave.”

The Middle East Institute, a nonpartisan think tank in Washington, accused Russia last week of violating an international agreement that allows Turkey to regulate transit of warships during times of war through the Turkish Straits, which connects the Black and Aegean seas.

Turkey triggered the Montreux Convention four days after Russia invaded Ukraine, which prevented Russia from bringing more warships to the Black Sea, but Moscow circumnavigated the agreement by using merchant vessels — which are not prohibited from the waterways — to supply its military operations in Ukraine, the institute found, citing an analysis of naval traffic.

The report also accused Russia of stealing Ukrainian grain “on an industrial scale” and using the profits to fund its war.

“It is absurd that Russia has been allowed to weaponize commercial trade by illegally blockading the Odessa and Chornomorsk harbors, while also profiting from the sale of stolen Ukrainian grain, exported from an occupied Ukrainian port,” the report’s authors wrote.

As the war entered its fourth month, there was little sign that the Zelensky administration or the Ukrainian people were prepared to negotiate an end that resulted in loss of Ukrainian territory.

A poll by the Kyiv International Institute of Sociology found that 82% of Ukrainians are not prepared to give up any land, even if it means the war continues.

Only 10% said they believed that giving up land was worth ending the conflict. Eight percent were undecided.

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The Washington Post’s Andrew Jeong in Seoul and Karen DeYoung, Karoun Demirjian and Timothy Bella in Washington contributed to this report.