A search is under way for the missing crew, with what is reported to be the first attack on shipping in the NATO member's economic zone.
Bulgarian Prime Minister Rumen Radev will make a statement on Tuesday after drones hit two commercial ships in the Black Sea off Bulgaria, sinking one, the government said.
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Radev was due to speak at 11:15am local time (10:15am CET) at the start of an emergency meeting on the strikes.
The crew of the sunken vessel are missing and those from the second ship have been rescued, public broadcaster BNR quoted Rumen Nikolov, head of rescue operations in Varna, as saying.
Bulgarian shipping news site Maritime.bg named the vessels as the Alfa Watan, which it said sank, and the Able, whose 18 crew abandoned ship after a fire broke out and were taken aboard the Bulgarian ferry Dioscuria, some of them injured.
The origin of the drones has not yet been established, it said.
Ship tracking website vesselfinder.com shows the Alfa Watan was sailing under the flag of Togo and was headed for the Romanian port of Sulina.
Its last known location was reported just after 2am CET on Tuesday approximately 70 nautical miles off the coast of Bulgaria.
The incident occurred just a day after a Turkish-owned cargo ship carrying grain in the Black Sea was attacked by at least one drone and ultimately sank after catching fire.
Our journalists are working on this story and will update it as soon as more information becomes available.