A Russian cargo ship sank early Saturday in the Black Sea off the Bulgarian coast, the Bulgarian transportation ministry said.
"The Russian Tolstoy cargo ship flying the North Korean flag sank about 04.00 local time 10-15 miles off the Bulgarian coast," Darik Radio quoted Minister of Transport Petar Mutafchiev as saying.
The ministry said that the crew reportedly comprised 13 members, including a Russian captain and they are all considered missing.
Bulgarian and Russian sea search-and-rescue teams, including a Bulgarian military helicopter, are attempting to find the survivors, but the rescue operation is hampered by bad weather.
A Bulgarian Navy frigate is expected to join the rescue effort later on Sunday.
Darik Radio said the 138-meter-long cargo vessel was carrying scrap metal from Rostov-on-Don in Russia to Nemrut in Turkey.
The ship has reportedly been in service for 37 years and was recently cited for numerous technical problems.