Drones reportedly struck two Turkish-owned commercial ships in Bulgaria's Black Sea Exclusive Economic Zone (EEZ) on Tuesday, sinking one of the vessels, Bulgarian authorities reportedly said.
The crew of the sunken ship is missing, while all 18 sailors aboard the second ship were rescued.
The attack was the first on ships in Bulgaria's exclusive economic zone, Bulgarian maritime outlet Maritime.bg reported.
Prime Minister Rumen Radev convened an emergency meeting with defense, interior and security officials.
The office of President Iliana Iotova confirmed the attack in the NATO member in an emailed statement, but did not provide details according to Reuters.
The strikes came a day after the Turkish-owned cargo ship Royad Mammadov sank off Romania, killing two crew members, amid reports of a drone attack.
The Black Sea has become increasingly dangerous for shipping amid drone warfare between Russia and Ukraine.
The Togo-flagged cargo ship Alfa Watan suffered critical structural damage and sank east of the Bulgarian coast.
No information is available on the fate of its crew.
"The crew of one ship is missing, while the sailors from the second vessel have been rescued," Rumen Nikolov, director of the Maritime Rescue Coordination Center in Varna, told Bulgarian public radio BNR.
The two ships sent distress signals at about 4 a.m. local time about 80 nautical miles off the Bulgarian coast, east of Cape St. Atanas and the town of Byala.
Around 9 a.m., one vessel disappeared from Border Police radar.
A serious fire broke out aboard the Palau-flagged cargo ship Able after the strike, forcing all 18 crew members to abandon the vessel. They were rescued by the Bulgarian-flagged ferry Dioscuria, which was diverted to the area. Some of the rescued sailors were injured and received first aid on board before being evacuated to hospitals on shore.
The origin of the drones remained unclear.
The office of Bulgarian President Iliana Iotova said two ships had been hit by drones in Bulgaria's Black Sea economic zone, without providing further details.
The 3,160-deadweight-ton Alfa Watan belongs to the fleet of Mersin-based Baraka Shipping, while the 7,416-deadweight-ton Able is part of the fleet of Istanbul-based Ocean Eagle.
Alfa Watan, built in 1976, was last recorded heading to the Romanian port of Sulina.
Radev's meeting includes representatives of the Defense, Interior, Transport, and Environment and Water ministries, the president's office, and the security services, the government information service said.
Radev was set to make a statement to the media at the start of the meeting.
On Monday, the Turkish-owned Royad Mammadov, carrying Ukrainian wheat to Italy, caught fire and sank off Romania, with Romanian media reporting it had been hit by a drone.
War-related objects found in the Black Sea, concentrated in Romanian, Bulgarian and Turkish waters, more than doubled in six months, maritime risk firm Ambrey said last month.