A merchant ship has sunk after drones hit two vessels off Bulgaria's Black Sea coast, Prime Minister Rumen Radev has said.
He indicated Russia was suspected of the attacks and that he had called in its ambassador.
One of the ships went down 17 nautical miles (31km) east of Krapets and one crew member was saved, he said. The other, Ukrainian-owned vessel was hit by three drones 18 nautical miles from the town of Byala and all 15 crew were rescued, he added.
The drone attacks come a week after two other commercial ships were hit by sea and aerial drones in Bulgaria's exclusive economic zone. One of the ships, the Alfa Watan, sank and all 10 crew, including nine Syrians, are presumed dead.
Radev convened an emergency meeting on Tuesday and said he had summoned Russian envoy Eleonora Mitrofanova. Although there was no conclusive evidence of blame on "either side... the logic suggests that the outcome benefits the Russian Federation", he said.
Bulgaria is a Nato member and the prime minister said he would speak shortly to secretary general Mark Rutte. Radev had earlier asked Nato and the EU for technical assistance in investigating the 6 October attacks, indicating it was unclear whether Russia and Ukraine were behind them.
Radev came to office during the summer and has since rejected suggestions that he is "pro-Russian".
Ukraine has directly blamed Russia for the latest attacks and European Commission President Ursula von der Leyen spoke on Tuesday of the Black Sea becoming "another front of escalation of the Russian war in Ukraine. This is unacceptable".
Russia's military has not publicly commented on the issue. It has recently intensified its strikes on vessels in the Black Sea, particularly the western side, as part of its war against Ukraine.
The same day that two ships were targeted off Bulgaria last week, Ukraine hit an oil tanker in the Russian Black Sea port of Sochi.
Moscow says it is targeting ships delivering weapons and dual-use cargo to Ukrainian ports - and cited attacks in the ports of Mykolaiv and Chornomorsk last week, along with one in the Black Sea.
However, Kyiv and its European allies say merchant vessels carrying grain and other civilian cargo are being attacked.
Meeting with his security chiefs on Tuesday, Bulgaria's prime minister said: "Another attack on merchant ships in the immediate vicinity of Bulgarian territorial waters is absolutely unacceptable".
Radev identified one of the hit vessels as the Maltese-flagged Chelsea 3, which is registered in Ukraine's Odesa, saying that five of its crew members were being treated for minor injuries. He gave no further details about the vessel that had sunk.
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".
"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.
Both Russia and Ukraine are major grain-exporting countries and Turkey has called on both countries to halt attacks in the Black Sea.
Moscow has repeatedly rejected calls by the US and Europe for a naval ceasefire in the Black Sea, blaming Ukraine for attacking Russian merchant ships in the area.
Russian President Vladimir Putin launched a full-scale invasion of Ukraine in 2022, and Moscow currently controls about 20% of Ukrainian territory.