Bulgaria's navy has warned of an object that looks like a sea mine, spotted south of Cape Emine on 21 October.
It was seen at 42°34.69′ north, 27°58.42′ east, at 8:00 am Coordinated Universal Time (UTC).
The warning is on the military's official website. It asks sailors to take care and keep at least five cables (about 0.5 nautical miles) from the object.
NATO has said it is stepping up surveillance and reconnaissance in the Black Sea after a series of drone attacks on ships in the region.
Bulgaria's air force is patrolling the country's territorial waters with F-16, MiG-29, Su-25 and L-39 aircraft, which watch and survey the area. The flights will go on in the coming days, the defence ministry said on Wednesday.