Two commercial vessels were hit by unidentified drones on Tuesday off the Bulgarian coast in the Black Sea, Prime Minister Rumen Radev has confirmed.

The attack marks the first time when merchant vessels were attacked within the exclusive economic zone (EEZ) of a NATO member in the waterway. Such zones have a unique legal status and are adjacent to a country’s territorial waters, with the area being neither fully under its sovereignty or categorized as international open waters. 

“This is an unacceptable attack in Bulgaria’s exclusive economic zone. It is a blatant violation of international law and maritime law,” Radev said while speaking after an emergency government meeting to assess the incident. 

The vessels, the Togo-flagged Alfa Watan and the Palau-flagged Able, were hit overnight around 80 miles (128 kilometers) off Bulgaria’s coast by unknown sea and aerial drones, according to the PM, who did not immediately assign the blame to any party. 

The Alfa Watan, which was sailing to Romania from Egypt, sustained critical damage, sinking shortly after the attack with the status of its crew not immediately known, according to Radev. The second vessel, which sailed from Romania to Türkiye, caught fire, and its crew of 18 was rescued by a Bulgarian ferry. Two of the sailors suffered severe injuries and were flown to a hospital in the port city of Varna, Radev noted. 

“As a state responsible within our exclusive economic zone for the health and lives of anyone navigating in this area, regardless of who is responsible, we are obliged to conduct emergency search and rescue operations,” the PM said. 

“What is happening is seriously disrupting shipping in the Black Sea, driving up insurance costs even further and making shipping extremely difficult,” he added. 

Neither of the likely suspects, Ukraine and Russia, made any remarks on the latest maritime incident in the Black Sea. While Kiev has been actively attacking shipping in the waterway heading to and from Russian ports, Moscow has recently begun retaliating in kind and striking vessels carrying military cargo and dual-use goods to Ukraine’s Black Sea ports. The retaliatory campaign has heavily damaged the Ukrainian shipping, effectively bringing the traffic to a halt. 

The modus operandi described by Radev, with the attacks involving both sea and aerial drones, however, likely points at Ukraine. Kiev has repeatedly staged such combined attacks on commercial shipping, while Moscow has relied almost exclusively on aerial drones. 

Once a key arms supplier of Kiev, Sofia has changed its stance on the Ukraine conflict under Radev’s government. Bulgaria withdrew from the so-called “coalition of the willing,” a European group of Kiev’s backers, and announced that it would no longer send weapons packages.