NATO chief Mark Rutte said on Tuesday the alliance would adjust its defences "as needed" after two ships were struck off Bulgaria's Black Sea coast in the latest such incident.

"NATO will continue to work closely with our allies in the Black Sea. 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," Rutte wrote online, after talking to Bulgaria's prime minister.

Meanwhile, European Commission President Ursula von der Leyen "strongly condemned" drone attacks on merchant ships near Bulgaria's coast.

"The Black Sea has become another front of escalation of the Russian war in Ukraine. This is unacceptable," von der Leyen said on X on Tuesday.

She said the attacks endangered crew members and threatened regional security, expressing solidarity with Bulgaria.

"Those responsible should be held accountable," she added.

Earlier on Tuesday, Bulgarian Prime Minister Rumen Radev said a distress call was received at 1.30 am (2230GMT) from the Maltese-flagged Chelsea 3, located 18 nautical miles east of Byala.

According to its crew, the Ukrainian-owned vessel, registered in Odessa, was struck on its deck and side by three aerial drones.

All 15 crew members, including seven Ukrainians and eight Azerbaijanis, were safely evacuated to the port of Varna, with five receiving medical assistance, according to the premier.

Radev said Bulgaria had informed its representatives to NATO and the EU and was seeking international support to investigate the drones' origin and strengthen maritime and air surveillance.

"We do not have clear and conclusive evidence to attribute responsibility to either side," he said, adding that he would summon the Russian ambassador for talks.

He also reported that another vessel was struck 17 nautical miles east of Krapets and subsequently sank, with one crew member rescued.