Two ships have been hit by drones off Bulgaria's Black Sea coast and one of them sank, in the second such incident in a week, Prime Minister Rumen Radev says.
The Ukrainian-owned Chelsea 3, sailing under Maltese flag, was hit by three drones about 30km off the Bulgaria coast early on Tuesday morning, Radev said.
Its crew - seven Ukrainians and eight Azerbaijanis - had been rescued.
Five crew members received medical assistance, but no one was seriously injured.
One crew member of the sunken ship had also been rescued, Radev said.
"This is absolutely unacceptable - another attack in the immediate vicinity of Bulgarian territorial waters," Radev said in a televised address.
"I am due to speak with NATO Secretary -General Mark Rutte."
Radev said there was no clear evidence who carried out the attack but "the outcome benefits the Russian Federation", adding that he would summon the Russian ambassador about the incident.
The attacks came after drone strikes sunk one cargo ship and damaged another in the Black Sea on October 6.
Twelve crew members remain missing after the ship sank, while the 18-member crew from the other vessel were rescued.
Bulgaria does not border either Ukraine or Russia, but as a NATO member it holds a strategic position on the Black Sea, where both countries have major ports.
There are growing concerns about hybrid attacks involving drones by Russia as it wages war in Ukraine.