Two ships were struck by drones off Bulgaria's Black Sea coast, one of which sank, Prime Minister Rumen Radev said.
According to Radev, the Ukrainian-owned, Malta-flagged vessel Chelsea 3 was attacked by three drones and damaged at around 1:30 a.m. Tuesday (2230 GMT Monday), about 18 nautical miles (33 km) off the Bulgarian coast.
Bulgaria's prime minister said the 15-member crew, made up of seven Ukrainians and eight Azerbaijanis, was rescued. Five sailors required medical support, though no one was seriously injured. He added that a sailor from the sunken ship was also rescued.
Radev called the incident "absolutely unacceptable."
"Another attack right next to Bulgaria's territorial waters," he said in a televised statement, adding that he would discuss the matter with NATO Secretary General Mark Rutte.
According to Radev, there is so far no clear evidence of who carried out the attack, but the "outcome benefits" Russia, and the Russian ambassador will be summoned to explain the incident.
A member of the NATO military alliance, Bulgaria holds a strategic position on the Black Sea, where both Russia and Ukraine have key ports.
The incident comes amid rising concerns over hybrid attacks linked to Russia.
Last week, drones struck two ships off Bulgaria's Black Sea coast, causing one to sink. All 10 sailors aboard the sunken vessel have still not been found.
