The Black Sea area has been hit hard by severe weather, with heavy snow causing disruption to transport in Romania and Bulgaria.
In Bucharest, the Romanian capital saw its two airports closed for several hours while Black Sea ports in Romania and Bulgaria were closed to vessels.
In the northeast of Bulgaria, a state of emergency was declared, with soldiers called in to help civil defence authorities clear roads and reach stuck motorists.
Viktoria Tarasova said that fierce icy wind and waves had forced search helicopters to abandon search efforts for the sunken vessel for the night, with the rescue mission not due back out until 0500 GMT.
Tarasova said: "The helicopters couldn't continue to operate in the dark. As for our boats, it's just impossible because they're all covered in ice.
"The winds are between 18 and 22 metres per second, with a squall of 25, the waves are three or four metres high and the chill factor is intense.
"Our rescue boats could end up in real difficulty themselves."
One lifeboat had already been forced to return to port because of the exceptionally icy conditions, in temperatures logged by the department at minus 9 degrees Celsius.
Doitchin Doitchinov, the ship's owner, told Bulgarian radio that captain Miroslav Dimitrov had reported a "powerful jolt to the ship's left side which caused it to crack".
As water flooded in, the skipper had contacted Ukrainian authorities who had advised that he launch the distress signal, with Russian coastguards the first to intervene.
Petar Moutaftchiev, the Bulgarian transport minister, said that the crew had transferred to two life-rafts.
Cargo of steel
The Bulgarian-registered Vanessa was carrying a cargo of just under 3,000 tonnes of steel from the Ukrainian port of Berdyansk when it sank in high winds in the Kerch strait, Tarasova said.
The strait divides Ukraine and Russia just north of the Black Sea.
Ten of the crew members were Bulgarian citizens and one was Ukrainian, she said.
One rescue vessel left the area of Thursday's sinking to bring the frostbitten sailor to shore for treatment.