Bulgarian Prime Minister Rumen Radev has summoned the Russian ambassador after two merchant ships were struck by drones off Bulgaria’s Black Sea coast and one of them sank.

Six crew members on the Guinea-Bissau-flagged HS Dignity were rescued and 10 others are missing, according to Defence Minister Dimitar Stoyanov, after it went down 17 nautical miles (31km) east of the village of Krapets having sent a distress signal.

The other ship, the Ukrainian owned Chelsea-3, was hit by drones 18 nautical miles from the town of Byala and all 15 crew were rescued.

The 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.

Mitrofanova was later quoted by Bulgarian media as telling Radev she had no information from Russia’s defence ministry relating to the latest attacks. She also said these strikes could have been a provocation by a third party.

Bulgaria is a Nato member, and Radev also spoke to secretary general Mark Rutte about the latest attacks.

“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 Dignity and Chelsea-3 were struck just outside Bulgarian territorial waters, in its Black Sea “exclusive economic zone”.

If this had happened just a few nautical miles to the west, Article 5 of the Nato treaty could have been triggered. The article defines an armed attack on one member of the military alliance as an attack on all.

“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.”

“We assume the drones are Russian,” he said, as it would have been “illogical” for Ukraine to target its own ships.

Rumen Radev had earlier asked Nato and the EU for technical assistance in investigating the previous attacks that took place on 6 October, indicating it was unclear who was behind them.

Radev came to office during the summer and although he has rejected suggestions that he is “pro-Russian”, his government has until now avoided blaming Moscow.

Ukraine has directly accused Russia of staging 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 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”.

While the search continued for the rest of the crew from the HS Dignity, Radev said five crew members on board the Chelsea-3 were being treated for minor injuries. The cargo ship is registered in the southern Ukraine port of Odesa.

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.