The deadliest body of water this year isn’t the Strait of Hormuz. A series of attacks in the Black Sea has killed more people than strikes on shipping so far in the war between Iran and the U.S., disrupting trade routes and turning up the heat on NATO.

Russian forces have targeted commercial ships carrying food exports from Ukraine out into the Black Sea. Ukraine, too, has attacked Russian shipping on the other side of the sea with drones.

In response, shippers have largely moved away from the waters off Ukraine and Russia to shelter from attacks. But the strikes have expanded to the waters of Romania and Bulgaria, both members of the North Atlantic Treaty Organization, in a spate of recent deadly incidents.

This year, there have been over 300 attacks in the Black Sea, according to figures from maritime security company Ambrey. The strikes have killed more than 30 seafarers and left more than 20 others missing, according to the Turkish Shipowners’ Association, MarineTraffic reports and local authorities.

That compares with around 130 attacks in the Strait of Hormuz and the wider Middle East, in which 25 seafarers have died, since the U.S.-Iran conflict began at the end of February, according to the International Maritime Organization.

In October, almost half of the attacks on a dozen vessels off Romania and Bulgaria left seafarers either dead, missing or injured.

Those attacks, on the edge of NATO territory, have been linked to Russia by some European leaders. Moscow has denied involvement.

They come alongside an increase in airspace violations, cyberattacks and acts of sabotage on the military alliance’s eastern border that European officials have linked to Moscow.

Romania and Bulgaria are particularly exposed to Russian hostility because their militaries and civil defense forces are among Europe’s least developed. NATO allies are deploying resources to assist in the region. On Monday, the European Commission announced plans to fast-track a Black Sea maritime-security hub in light of the recent attacks there, but didn’t name Russia as behind them.

The Black Sea holds significant strategic importance as a link between Europe and Asia. Six countries—Bulgaria, Georgia, Romania, Russia, Turkey and Ukraine—border its shores, giving it access to vital international trade routes.

For months after Russia’s large-scale invasion of Ukraine in 2022, the Black Sea was a focus of naval warfare and attacks on commercial shipping, but gradually hostilities there ebbed. Russian warships retreated to port and, under international pressure, stopped targeting merchant vessels carrying grain and other foodstuffs from Ukraine. Countries across Africa and parts of the Middle East depend on Ukraine’s harvests to feed their people.

The recent increase in violence has sharply reduced commercial activity in the Black Sea. The number of ships entering via the Bosporus fell 41% in October compared with the same month in 2025, and transits in both directions declined 38% in that period, according to Global Fishing Watch, a nongovernmental organization that tracks maritime activity.

Graphics sources: Global Fishing Watch (vessel presence); Ambrey (reported attacks); MarineTraffic (vessel positions); Marine Regions (maritime boundaries); SAHEC (hybrid incidents); Institute for the Study of War (Russian forces)

Write to Daniel Kiss at daniel.kiss@wsj.com and Daniel Michaels at Dan.Michaels@wsj.com