A merchant ship has sunk after drone attacks struck two vessels off Bulgaria's Black Sea coast, Prime Minister Rumen Radev announced on Tuesday.

One ship went down 17 nautical miles east of Krapets. One crew member was rescued. The other vessel, the Ukrainian-owned Chelsea 3, sailing under a Maltese flag, was hit by three drones approximately 18 nautical miles from the town of Byala at around 1:30am local time. All 15 crew members were rescued, five of whom received medical treatment for minor injuries. The crew included seven Ukrainians and eight Azerbaijanis.

Radev convened an emergency meeting and summoned Russian Ambassador Eleonora Mitrofanova. While he said there was no conclusive evidence pointing to blame on "either side," he added that "the logic suggests that the outcome benefits the Russian Federation."

Mitrofanova was later quoted by Bulgarian media as telling Radev she had no information from Russia's defence ministry regarding the attacks. She also suggested the strikes could have been a provocation by a third party.

Russia's military has not publicly commented on the incident.

Radev described the attacks as "absolutely unacceptable," saying: "Another attack on merchant ships in the immediate vicinity of Bulgarian territorial waters is absolutely unacceptable." He confirmed he would speak with NATO Secretary-General Mark Rutte.

European Commission President Ursula von der Leyen condemned the attacks, saying the Black Sea had become "another front of escalation of the Russian war in Ukraine. This is unacceptable." She added: "I strongly condemn the drone attacks on merchant ships near Bulgaria's coast. Putting the lives of the crew in grave danger and threatening security in the Black Sea. We stand with Bulgaria and thank its rescuers for their swift response. Those responsible should be held accountable."

Ukraine directly blamed Russia for the attacks. Ukrainian Foreign Minister Andrii Sybiha said "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." He added: "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."

The incident follows a similar attack one week earlier, when drones struck two other commercial ships in Bulgaria's exclusive economic zone. One of those vessels, the Alfa Watan, sank. All 10 crew members, including nine Syrians, remain unaccounted for and are presumed dead.

On the same day as last week's attacks, Ukraine struck an oil tanker in the Russian Black Sea port of Sochi.

Moscow has stated it targets ships delivering weapons and dual-use cargo to Ukrainian ports, citing strikes on the ports of Mykolaiv and Chornomorsk. Ukraine and its European allies maintain that merchant vessels carrying grain and other civilian cargo are being attacked.

Moscow has repeatedly rejected calls for a naval ceasefire in the Black Sea, blaming Ukraine for attacking Russian merchant ships in the area. Turkey has called on both sides to halt attacks in the Black Sea.

Bulgaria is a NATO member and holds a strategic position on the Black Sea. Radev had previously requested NATO and EU technical assistance in investigating the attacks from the previous week. He has rejected suggestions that he is pro-Russian since taking office in the summer.

Russia launched a full-scale invasion of Ukraine in 2022 and currently controls approximately 20 percent of Ukrainian territory.