Two cargo ships were hit by drones overnight and one was sunk, Bulgarian officials said on Tuesday.
Bulgaria stopped short of explicitly accusing Russia of carrying out the attacks but Prime Minister Rumen Radev said their outcome "benefits the Russian Federation".
There was no immediate comment from Moscow on the incidents.
"This is absolutely unacceptable - another attack in the immediate vicinity of Bulgarian territorial waters," Radev said in a televised address.
The attacks were also condemned by Ukraine and the EU.
"The Black Sea has become another front of escalation of the Russian war in Ukraine," European Commission President Ursula von der Leyen said on X.
"Those responsible should be held accountable."
Ukrainian Foreign Minister Andrii Sybiha said on X on Tuesday that the strike "once again demonstrates Moscow's deliberate disregard for civilian lives".
"The Black Sea is a lifeline for global food security. Russia is attacking it to weaponise hunger and hold the world hostage," he said.
Speaking with von der Leyen and NATO Secretary-General Mark Rutte by telephone on Tuesday, Radev urged their support to bolster his country's maritime and airspace surveillance, the Bulgarian government said in a statement.
NATO would continue to adjust its military presence on its eastern flank "as needed", Rutte said.
Bulgarian officials said five crew members had been rescued but 11 were still listed as missing from the ship which sank off the coast of the village of Krapets.
President Iliana Iotova identified the vessel as the Guinea-Bissau-flagged cargo ship HS Dignity and said a search-and-rescue operation was ongoing.
Another ship, the Chelsea 3 sailing under a Maltese flag, was also hit by three drones overnight, Bulgarian officials said.
All crew from that ship - seven Ukrainians and eight Azerbaijanis - were rescued and none seriously injured.
A week ago nine Syrian sailors and one Egyptian were lost at sea from a ship that sank when hit by drones in Bulgaria's Black Sea exclusive economic zone.
Another ship was also hit that night but the crew was rescued.
After meeting Radev in Sofia, Russian ambassador Eleonora Mitrofanova said she had no information from the Russian defence ministry about attacks on commercial vessels in the Black Sea but did not rule out they could be a provocation by a third party, the Bulgarian government said.
