Russia escalated its war on Ukraine’s grain exports again this week, sending wheat prices soaring and threatening to exacerbate global hunger as it seeks to blockade one of the world’s foremost breadbaskets.

After withdrawing from a U.N.-brokered grain deal on Monday, which allowed ships carrying grain from Ukrainian ports to reach world markets, the Russian Defense Ministry on Wednesday announced that any vessels en route to Ukrainian ports would be regarded as potentially carrying military cargo—and could be subject to attack in what would amount to a significant escalation of the conflict and a challenge to long-standing U.S. efforts to ensure freedom of navigation around the globe.

The threat came as Ukraine’s port city of Odesa has been subjected to a nightly barrage, for four days straight now, of Russian missile strikes that may have impacted up to 60,000 tons of grain, warehouses, and dock facilities, prompting top U.S. officials to sound the alarm on potential food shortages. On Friday, Ukrainian officials said Moscow’s latest strike had destroyed 100 tons of peas and 20 tons of barley. The White House also warned that U.S. intelligence indicated that Russia had laid sea mines on the approach to Ukrainian ports, intending to blame Kyiv as part of a false-flag operation.

“This is yet another twist and turn by [Russian President Vladimir] Putin and Russia to broaden the war, so to speak, and to include the maritime domain,” said Sebastian Bruns, an expert in maritime strategy and security at the Institute for Security Policy at Kiel University. “He’s looking for other ways to intimidate the West.”

Russia’s threats pose a major test for the international community’s ability to safeguard shipping in the area and freedom of navigation, a cornerstone of maritime law that allows vessels to sail unimpeded through international waters. While Russia has waged war against Ukraine for over a year, an attack on a single commercial vessel could embroil several countries in one fell swoop, given the globalized nature of shipping.

“If somebody attacks those civilian vessels, it’s a throwback to World War I and the sinking of the Lusitania,” said retired Adm. James Foggo, who served as the commander of U.S. naval forces in Europe and Africa, referring to the British passenger ship sunk by a German U-boat in 1915. “It’s a slippery slope to a much more serious conflict.”

The U.S. Navy routinely conducts freedom of navigation operations around the world to preserve the right to access international waters. On Monday, U.S. Defense Secretary Lloyd Austin dispatched U.S. fighter jets and the guided-missile destroyer USS Thomas Hudner to the Middle East in response to the Iran’s recent attempts to seize merchant shipping vessels in the region. But in the Black Sea, where Russia and Western powers have vied for influence for centuries, Washington and its NATO allies are constrained by geography, geopolitics, and paperwork.

The Black Sea, which is slightly larger than the U.S. state of California, is ringed by six countries. Half of them—Romania, Turkey, and Bulgaria—are NATO members. The narrow entrance to the sea, the Bosporus, is controlled by Turkey under a 1936 agreement known as the Montreux Convention. In the early days of Russia’s invasion, Ankara invoked the convention to close off the strategic choke point to warships, undercutting Russia’s ability to reinforce its Black Sea Fleet. Washington withdrew its warships from the region shortly before the invasion as it has sought to avoid a direct confrontation with Russia.

“If this was the South China Sea, we would be conducting freedom of navigation operations,” said Kurt Volker, who served as a U.S. ambassador to NATO during the George W. Bush administration. “Because of the unwillingness to confront Russia, we are backing off from this principle, which we have otherwise stood by.”

On Monday, U.S. National Security Council spokesperson John Kirby said there were no plans under consideration to escort commercial shipping in the area in the wake of Russia’s withdrawal from the grain deal. In the late 1980s, the Navy did escort reflagged Kuwaiti oil tankers out of harm’s way in the Persian Gulf during the closing stages of the Iran-Iraq War, a period known as the “Tanker War.”

“The U.S and its allies mustered significant resources to support the free flow of international cargo [during the Tanker War],” Bruns said. “It helped the U.S. Navy, and the U.S. at large, to play a significant role as guardian of the maritime system.”