Bulgaria has stepped up diplomatic and security efforts after drone strikes hit two merchant vessels near its Black Sea coastline, sinking one ship and leaving 10 sailors missing in an escalation that has raised fresh fears over the safety of commercial shipping and the spread of the Russia-Ukraine conflict.
The attacks prompted Prime Minister Rumen Radev to summon Russian Ambassador Eleonora Mitrofanova as Bulgarian authorities investigated the incidents and sought answers over who was responsible.
The first vessel, the Guinea-Bissau-flagged HS Dignity, sank after sending a distress signal, with six crew members rescued and 10 others still missing.
Defence Minister Dimitar Stoyanov said the ship went down about 17 nautical miles, or 31 kilometres, east of the village of Krapets.
A second vessel, the Ukrainian-owned Chelsea-3, was struck by drones about 18 nautical miles from the coastal town of Byala. All 15 crew members were rescued, although five sustained minor injuries and were receiving treatment, according to Radev.
The latest strikes have increased concern in Sofia because they follow another series of attacks on commercial vessels in Bulgaria's exclusive economic zone a week earlier. One of the ships involved in those incidents, the Alfa Watan, sank, with all 10 crew members, including nine Syrians, presumed dead.
The repeated attacks have put pressure on Bulgarian officials to establish who is targeting merchant ships operating near the country's coastline and whether the incidents pose a wider threat to regional security.
Bulgaria presses Russia for answers
Radev convened an emergency meeting with his security officials on Tuesday before summoning Mitrofanova over the latest attacks.
Although he acknowledged that there was no conclusive evidence establishing responsibility, the Bulgarian leader said the apparent consequences of the incidents pointed towards Russia's interests.
"There was no conclusive evidence of blame on "either side... the logic suggests that the outcome benefits the Russian Federation", he said.
Mitrofanova later told Bulgarian media that she had no information from Russia's defence ministry concerning the latest strikes. She also suggested that a third party could have organised the attacks as a provocation.
Bulgaria's defence minister, however, said Russian involvement was suspected, arguing that Ukraine would have little reason to target ships associated with its own side.
"We assume the drones are Russian," he said, as it would have been "illogical" for Ukraine to target its own ships.
The minister warned that the repeated incidents were bringing the dangers of the war closer to Bulgarian waters and increasing the risks facing vessels using the Black Sea.
"Escalation in the Black Sea is steadily increasing," Bulgaria's defence minister later told reporters after speaking to his counterparts in Romania and Turkey. "We see this conflict drawing ever closer to our territorial waters."
Bulgaria has not established who carried out the latest attacks, and Russia's military had not publicly commented on the incidents at the time of the report.
Nato monitors growing maritime threat
The two vessels were hit outside Bulgaria's territorial waters but within its exclusive economic zone, an area where the country has specific rights over marine resources and related economic activities.
Their location has drawn attention to the security implications for Bulgaria, a member of the North Atlantic Treaty Organization (Nato), as the conflict increasingly affects commercial shipping across the Black Sea.
Had the attacks taken place a few nautical miles farther west, within Bulgarian territorial waters, they could have raised more direct questions about Nato's collective defence obligations.
Article 5 of the alliance's treaty provides that an armed attack against one member is considered an attack against all, although its application depends on the circumstances of an incident.
Following the latest strikes, Radev spoke to Nato Secretary General Mark Rutte about the security situation and the need to protect the alliance's members along the Black Sea.
Rutte said Nato would maintain close cooperation with its allies in the region and continue assessing the security environment.
"Nato will continue to work closely with our allies in the Black Sea," Rutte said after the talks.
"We have already strengthened our posture along the eastern flank and will continue to monitor the security environment, adjusting our deterrence and defence posture as needed," he added.
The Bulgarian prime minister had previously asked Nato and the European Union for technical assistance in investigating the earlier attacks on commercial vessels on October 6. At the time, the authorities had not established who was behind the strikes.
Radev, who took office during the summer, has rejected claims that he is "pro-Russian". His government had until now avoided directly blaming Moscow for the earlier incidents, despite growing concern over the risks posed by attacks near Bulgarian waters.
Ukraine accuses Moscow of targeting civilian shipping
Ukraine has directly accused Russia of carrying out the latest attacks, while European Commission President Ursula von der Leyen warned that the Black Sea was becoming another area of rising tension linked to the war.
"The Black Sea is becoming another front of escalation of the Russian war in Ukraine. This is unacceptable," she said on Tuesday.
The incidents come as Russia intensifies strikes against vessels in the Black Sea, particularly in its western waters, as part of its wider military campaign against Ukraine.
Moscow maintains that it is targeting ships carrying weapons and goods that can serve both civilian and military purposes to Ukrainian ports. Russian officials have cited attacks on the ports of Mykolaiv and Chornomorsk last week, as well as another incident in the Black Sea, to support their position.
Ukraine and its European allies, however, accuse Russia of striking commercial vessels transporting grain and other civilian goods, raising concerns over the safety of international shipping routes.
The conflict has also seen attacks on Russian-linked maritime targets. On the same day that two commercial vessels were targeted off Bulgaria last week, Ukraine struck an oil tanker in the Russian Black Sea port of Sochi.
The exchange of attacks has increased pressure on countries around the Black Sea to prevent further escalation and protect ships carrying goods through the region.
Turkey has called on both Russia and Ukraine to stop attacks on vessels operating in the Black Sea. Moscow has repeatedly rejected calls by the United States and European countries for a naval ceasefire, accusing Kyiv of targeting Russian merchant ships.
Kyiv warns of consequences for global food security
Ukrainian Foreign Minister Andrii Sybiha condemned the latest attacks, accusing Moscow of putting civilian lives and maritime security at risk.
"Russia's latest attack on commercial vessels near Bulgaria's territorial waters... once again demonstrates Moscow's deliberate disregard for civilian lives and maritime security in the Black Sea".
Sybiha said the attacks were particularly concerning because the Black Sea is a key route for international food shipments, including grain exported by major agricultural producers.
"The Black Sea is a lifeline for global food security. Russia is attacking it to weaponise hunger and hold the world hostage. This must be met with strength," he added.
Russia and Ukraine are both major grain-exporting countries, making the security of Black Sea shipping routes important to international food supplies. Continued attacks on commercial vessels have therefore raised concerns beyond the countries directly involved in the war.
Radev also criticised the latest incidents, telling his security chiefs that the targeting of merchant ships so close to Bulgaria's territorial waters was unacceptable.
"Another attack on merchant ships in the immediate vicinity of Bulgarian territorial waters is absolutely unacceptable".
As rescuers continued searching for the 10 missing crew members from the HS Dignity, Bulgarian authorities faced renewed pressure to determine the circumstances surrounding the attacks and assess the risks to vessels operating near the country's coast.
The incidents have added to fears that the maritime dimension of the Russia-Ukraine war is spreading further across the Black Sea, threatening civilian shipping and placing neighbouring countries under growing pressure to strengthen their security measures.
Russian President Vladimir Putin launched a full-scale invasion of Ukraine in 2022. Moscow currently controls about 20% of Ukrainian territory.