A surge in attacks on cargo ships in the Black Sea is emerging as a growing security challenge for NATO, with Romania and Bulgaria increasingly exposed to the escalating maritime conflict between Russia and Ukraine.
After seven strikes on cargo ships near the two NATO members last month, the alliance announced plans to deploy maritime patrol aircraft, radars and drones to strengthen its eastern flank in the Black Sea.
However, attacks continued. On October 23, another ship was struck just outside Romanian territorial waters, while a Greek container ship was sunk by naval drones in Bulgaria's exclusive economic zone last week.
Military analysts said they had little doubt that Russia was behind the attacks. A naval drone seen in video footage of the latest strike resembled a Russian variant, according to shipping security firm Ambrey.
"I think it's undeniably an escalation," said Sidharth Kaushal, a senior fellow at the London-based Royal United Services Institute (RUSI) think-tank.
Russia has denied targeting Ukraine's neighbours with drones, accusing European governments of escalating the war by supplying military assistance to Kyiv.
Many of the ships hit in October were anchored near or travelling to and from a key Romanian canal used by Ukraine for exports. One was struck in Romanian territorial waters, while eight others were in the exclusive economic zones of Romania and Bulgaria.
Iulia Joja, director of the Black Sea programme at the Washington-based Middle East Institute, said Russia was testing Western limits by carrying out attacks while anticipating a limited response from NATO.
A NATO official described Russian actions aimed at discouraging alliance members from supporting Ukraine as a serious security threat, reiterating that the alliance's commitment to defending all its members, including those along the Black Sea, remained firm.
The official said NATO needed cost-effective counter-drone capabilities and had committed to investing more than $40 billion in such systems over the next five years.
The Black Sea is a crucial trade route for Romania and hosts its Neptun Deep offshore gas project, which is expected to make the country the European Union's largest gas producer next year. A naval drone approached the project in August.
Bucharest has accused Russia of carrying out systematic attacks on civilian vessels but has stopped short of attributing individual strikes, citing the need for forensic evidence. Bulgaria has not directly blamed Moscow but said some attacks appeared to benefit Russia.
The attacks have intensified as Washington reassesses its military presence in Europe and shifts its strategic focus towards China. The United States withdrew 1,000 troops from Romania last year.
A senior Romanian official said the situation presented an opportunity for Russian President Vladimir Putin to exploit divisions in the transatlantic alliance, as NATO and EU attention remained focused on other regions, particularly the Baltic states.
Romania has called for more surface-to-air interceptors and radar systems to counter the growing threats.
The escalation has disrupted a major Black Sea shipping corridor used by Russia and Ukraine to export grain and oil.
Between May and October, the two countries struck 317 merchant and other vessels in the Black Sea and Sea of Azov, more than four times the 74 vessels hit during the previous six months, according to Tomas Alexa, a senior analyst at Ambrey Analytics.
Ukraine carried out 137 attacks on ships in the third quarter, compared with 92 by Russia. However, the pattern reversed in October, when Russia launched 38 strikes against vessels, compared with five by Ukraine, Ambrey data showed.
The attacks have damaged ports and sharply reduced exports, forcing both countries to seek alternative trade routes.
Russia exported only 300,000 tonnes of grain through the Black Sea and Sea of Azov in September, down from 4.74 million tonnes in the same month last year, according to Russian export data seen by Reuters.
Ukraine suspended operations at its Black Sea ports in late July following Russian attacks, redirecting shipments through Danube river ports and Romania's Sulina Canal.
Despite the alternative routes, Ukraine's grain exports through its ports fell to 810,000 tonnes in September, compared with 2.1 million tonnes in the same month last year, government data showed.
Andriy Ryzhenko, a naval analyst and former senior Ukrainian naval officer, said Russia's primary objective was to further restrict Ukraine's exports, while testing NATO's response was a secondary goal.
The latest attack took place as defence ministers and military chiefs from Romania, Bulgaria and Turkey met in Sofia to discuss security in the Black Sea.
Turkey, which controls the passage of warships through the Bosphorus and Dardanelles under the Montreux Convention, has been seeking responses from Moscow and Kyiv to a proposal for a limited maritime ceasefire.
Several countries, including Egypt, Turkey and India, have also called for a maritime truce to prevent further disruption to shipping as global fuel and grain prices remain high.
US Secretary of State Marco Rubio said on Saturday that international interest in a limited ceasefire was growing.
However, Russia has rejected the proposal, arguing that it would give Ukraine an opportunity to recover.
Ukrainian President Volodymyr Zelensky has warned that continued attacks on shipping could trigger an agricultural crisis.
"Everyone is talking about diesel and forgetting about grain. In a month or two, everyone will start talking about an agricultural crisis," he said last month.