Two ships were struck off the Bulgarian coast on Tuesday, Prime Minister Rumen Radev said, adding that he would summon the Russian ambassador over the latest such incident in the Black Sea.
Radev wrote on social media that a Ukrainian-owned cargo ship was "hit by three aerial drones", and shortly afterwards another ship was hit and sank.
"We lack solid, clear evidence to level accusations against any specific party; however... logic suggests the outcome works to the advantage of the Russian Federation," he wrote.
Radev identified the first vessel as the Malta-flagged cargo ship Chelsea 3 and said all 15 crew members -- seven Ukrainians and eight Azerbaijanis -- had been rescued.
He said a second vessel was "struck" and one crew member had been rescued, but he gave no further details of the incident.
"This latest attack on commercial vessels in the immediate vicinity of Bulgarian territorial waters is absolutely unacceptable," Radev said.
Bulgaria will summon the Russian ambassador over the attacks, he said, and would also discuss them with NATO chief Mark Rutte.
EU chief Ursula von der Leyen condemned the attacks, calling them "another front of escalation of the Russian war in Ukraine".
"We stand with Bulgaria and thank its rescuers for their swift response. Those responsible should be held accountable," she posted on social media.
Ukraine's Foreign Minister Andriy Sybiga also blamed Russia, denouncing what he called Moscow's "deliberate disregard for civilian lives and maritime security in the Black Sea".
"The Black Sea is a lifeline for global food security. Russia is attacking it to weaponise hunger and hold the world hostage," he posted on X.
- Conflict spreads -
Amid mounting fallout from Russia's 2022 invasion of Ukraine, drone attacks have become increasingly frequent in recent months across eastern European countries.
The conflict has also spread across the eastern Black Sea, with at least 45 merchant vessels attacked over the past 12 months in areas of the Black Sea previously not listed as high risk, the maritime risk management group Ambrey said last month.
No strikes were recorded in the areas in the preceding 12 months, Ambrey said, adding that the increase was driven by attacks further from the coasts of Russia and Ukraine.
Last week, two commercial vessels were hit in the Black Sea off Bulgaria in an attack widely blamed on Russia, though Bulgaria said the circumstances were unclear.
The dual strike came a day after another ship caught fire and sank off the Romanian coast, killing one person and leaving another missing. Ukrainian President Volodymyr Zelensky said the ship was hit by Russian drones.
Bulgaria, an EU and NATO member, said Monday that it had requested technical assistance to determine the origin of the series of attacks.
In letters sent to the leaders of both organisations, Radev requested the issue be examined as a priority and also said Bulgaria had requested assistance from the European Maritime Safety Agency.
AFP