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

Eleven crew members are missing and one sailor was saved, according to local officials, after one of the vessels sank 17 nautical miles (31km) east of the village of Krapets.

The other vessel, Ukrainian owned, 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 two ships were struck just outside Bulgarian territorial waters.

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.

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 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.