As families strolled along a promenade in the Ukrainian port city of Odesa recently, a lone cargo ship sailed in the distance. Suddenly, a drone dropped out of the clear blue sky and slammed into the vessel, sending smoke billowing across the horizon.

The attack, which I witnessed in late July, was part of a Russian campaign targeting cargo vessels that has halted nearly all shipping in and out of Odesa, Ukraine’s most crucial link to the world economy.

Coming at the start of the harvest season, the Russian campaign in Ukraine, which is a major producer of grains like wheat, corn and barley, could throw global food supply chains into turmoil. In 2022, during the first months of the full-scale war, Russia established a blockade of Ukrainian ports, causing a spike in global food prices and placing an estimated 70 million people at increased risk of acute food insecurity.

“Russia is hunting these vessels,” Dmytro Barinov, president of the Ukrainian Ports Association, said in an interview. “It’s a professional hunting for civilian vessels.”

The Russian strikes are the latest chapter in a long-running battle over the Black Sea and the adjacent Sea of Azov. New weapons that allow for precision targeting at great distances have made slow-moving cargo vessels easy targets. In June and July, Russian forces attacked at least 50 cargo ships, according to Ukrainian officials, killing more than 30 civilians.

Ukraine has also systematically targeted Russian-linked tankers and cargo vessels. Moscow’s maritime campaign appears to be a response to Ukraine’s decimation of the Russian fleet of river-sea oil tankers in the Sea of Azov and to Kyiv’s continued targeting of ships transporting Russian goods, which has left occupied Crimea isolated and starved for fuel.

Ukraine’s latest major assault took place before dawn on Wednesday, when the country’s forces attacked the Russian port of Novorossiysk, on the Black Sea, with jet-powered drones, missiles and unmanned naval systems, President Volodymyr Zelensky said. Air defense positions, piers and seaport infrastructure were struck, he said.

Three people died, including an 8-year-old child, and about 60 residential buildings were damaged, Veniamin Kondratyev, the governor of the Krasnodar region, which includes Novorossiysk, said on social media. Andrei Kravchenko, the mayor of the city, which is a critical hub for grain exports, declared a state of emergency. He said the city’s water supply had been suspended after two mains were damaged.

The escalating battle between Russia and Ukraine is a flashpoint in a larger global phenomenon that threatens one of the foundational tenets of the international order: freedom of navigation.

That maritime system, which underpins modern global commerce, is under strain as state and nonstate actors increasingly use drones, anti-ship missiles and mines to disrupt routes, particularly in places like the Red Sea and the Strait of Hormuz. The strait, a key conduit for oil and gas, has effectively been closed since the United States and Israel attacked Iran in February.

At the port in Odesa and two others nearby, the Russian blockade could trap millions of tons of grain. The three ports account for about 60 percent of Ukraine’s total monthly export revenue of $3.5 billion. The Ukrainian Agri Council, a trade group, has warned of a potential wave of bankruptcies and has pleaded for emergency loans and state guarantees.

The situation has echoes of the 2022 Russian blockade on Ukrainian ports. In July that year, the United Nations and Turkey brokered the Black Sea Grain Initiative, which allowed shipping to resume. The deal fell apart a year later. But by that point, Ukraine had driven Russian warships out of the northwestern part of the Black Sea and was largely able to continue shipping.

As Russia has stepped up strikes in recent weeks, among the dozens of ships hit was a bulk carrier called the Golden Leo.

Blesson Sebastian, a 28-year-old second officer on the Golden Leo, grew up in Chennai, India, dreaming of the sea, he said. He had been to Ukraine many times over the past eight months, but when his vessel arrived last month, he said, the pace of Russian attacks was far more intense than anything he had experienced on previous trips.

The crew avoided Odesa’s main port, docking at nearby Chornomorsk. It offered no sanctuary. A neighboring ship was bombed by drones, killing an officer and a security guard, Mr. Sebastian said. The Golden Leo’s crew slept in steel bunkers.

On July 19, the ship was fully loaded with grain and departed. Mr. Sebastian said he saw families enjoying a peaceful afternoon at the beach as the boat pulled away. He told nervous younger crew members that everything would be fine.

At 4:30 p.m., the ship’s Syrian captain went to his cabin to pray, while Mr. Sebastian sat on the bridge reading his Bible.

Once the boat was four miles out from the port, Mr. Sebastian said, an air alarm wailed.

Volodymyr Mykhailov, a Ukrainian who was onboard to pilot the boat out of the harbor, advised leaving the bridge because Russian drones typically target the navigation deck, Mr. Sebastian said.

As Mr. Mykhailov and Mr. Sebastian made their way down the stairs, a Russian missile slammed into the hull.

“It rained fire,” Mr. Sebastian told me in an interview near the port. “This is it,” he recalled thinking. “It’s the end.”

Mr. Mykhailov was thrown into Mr. Sebastian, and they both tumbled to the ground. Deafened by the blast, Mr. Sebastian shook Mr. Mykhailov, but he was unresponsive. He died in Mr. Sebastian’s arms.

Stumbling through shattered steel, Mr. Sebastian found the captain severely burned and the boatswain unconscious. Blinding smoke poured from the engine room. The few survivors scrambled aboard a small boat that had been intended to return Mr. Mykhailov to port after his piloting duties.

As the survivors escaped, they watched the Golden Leo burn “like a furnace,” Mr. Sebastian said.

Nine crew members and Mr. Mykhailov were killed in the strike. The boat sank a week later.

Elisabeth Braw, an expert on maritime threats who works at the Atlantic Council, a think tank, said that Russia was waging a systematic and indiscriminate campaign against merchant vessels carrying agricultural goods and other nonmilitary cargo in and out of Ukrainian ports.

Such attacks violate treaties such as the United Nations Convention on the Law of the Sea, she said.

While the Russian-linked oil tankers and cargo vessels that Ukraine has targeted are technically civilian ships, maritime experts said, Kyiv attacks them because they are used to circumvent sanctions and export energy that funds the Kremlin’s war effort.

Western nations have at times intercepted such ships, part of Russia’s so-called shadow fleet. On Wednesday, during a visit aboard a Russian warship in the Pacific, President Vladimir V. Putin called those efforts “piracy and robbery” and vowed “to respond in kind.”

Russian operators often direct drones to loiter a few miles off the Ukrainian coast, where they “seek out” merchant vessels, Sr. Lt. Oleksandr, the commander of Ukraine’s Ninth Patrol Boat Division, said in an interview. The Russian drones also targeted rescuers after strikes, said the commander, who requested that his last name be withheld in accordance with Ukrainian military protocols.

After the missile hit the Golden Leo, Mr. Sebastian suffered first-degree burns. Like many colleagues stuck in dangerous seas around the world, he said, he was done sailing.

“Money is not bigger than life,” he said.

Liubov Sholudko and Paul Sonne contributed reporting.