LONDON/BUCHAREST:  Repeated attacks on commercial ships in the Black Sea are increasing pressure on NATO, raising concerns that the maritime conflict between Russia and Ukraine could spread closer to alliance territory and threaten the security of its eastern members, particularly Romania and Bulgaria.

After seven attacks on cargo ships near the two NATO countries last month, the US-led alliance announced plans to strengthen its eastern defenses in the Black Sea. The measures include deploying maritime patrol aircraft, radar systems and drones.

However, the attacks continued. Three days later, on October 23, another ship was hit just outside Romanian waters. Last week, a Greek container ship was sunk by naval drones inside Bulgaria's exclusive economic zone.

Military analysts said Russia almost certainly carried out the attacks. Shipping security company Ambrey said a naval drone seen in video footage of the latest attack appeared similar to a Russian model.

Sidharth Kaushal, a senior fellow at the London-based RUSI think tank, described the developments as a clear escalation.

Russia has denied targeting Ukraine's neighboring countries with drones. Moscow has instead accused European governments of escalating the war by providing military assistance to Ukraine. Russia's Defense Ministry did not respond to a request for comment.

Many of the ships attacked in October were anchored near, or traveling to and from, an important Romanian canal Ukraine uses to export goods. One vessel was struck inside Romanian territorial waters, while eight others were in the exclusive economic zones of Romania or Bulgaria.

Iulia Joja, director of the Black Sea program at the Washington-based Middle East Institute, said Russia was exploiting Western concerns about escalating the conflict.

She said the attacks were meant to test NATO's limits, knowing the alliance would try to avoid strong military action. She compared the strategy with previous Russian drone incursions into NATO airspace.

A NATO official said Russian actions aimed at discouraging alliance members from supporting Ukraine represented a serious security threat. The official stressed that NATO's commitment to protecting all its members, including those along the Black Sea, was "ironclad".

The official added that affordable systems to counter drones were urgently needed. NATO has already committed more than US$40 billion to developing and purchasing anti-drone equipment over the next five years.

Black Sea a Crucial Waterway

The Black Sea is particularly important to Romania because of its role in international trade and its offshore Neptun Deep gas project. The project is expected to make Romania the European Union's largest gas producer next year. A naval drone approached the Neptun Deep site in August.

Bucharest has accused Russia of carrying out systematic attacks on civilian ships but has avoided blaming Moscow for individual incidents without sufficient evidence. Bulgaria has also stopped short of directly accusing Russia, although it has said some attacks appear to benefit Moscow.

The attacks come as the United States reviews its military presence in Europe and shifts greater attention toward China. Washington withdrew 1,000 troops from Romania last year.

While NATO has strengthened its military presence in the Baltic countries and Poland, Joja said it had not taken similar action in Romania despite requests from Bucharest.

She questioned whether Brussels and Washington had fully recognized the Black Sea as part of NATO's security commitments.

Countries including Egypt, Turkey and India have supported efforts to establish a maritime ceasefire to protect shipping as disruptions have pushed up global fuel and grain prices.

U.S. Secretary of State Marco Rubio said on October 31 that international support for a limited ceasefire was growing.

Russia has rejected the proposal, arguing that it would allow Ukraine to recover. Ukrainian President Volodymyr Zelenskyy has warned that the disruption to grain exports could lead to a wider food crisis.

"Everyone is talking about diesel and forgetting about grain. In a month or two, everyone will start talking about an agricultural crisis," Zelenskyy said in October.