Seven of the 11 crewmembers of Bulgarian cargo ship Vanessa, which foundered in the Azov Sea early on Thursday, are dead, Darik Radio quoted Russian rescue officials as saying.
One sailor, mechanic Nikolay Dimitrov, has been rescued during the day and taken to a hospital in Russia's Kavkaz port, with his life out of danger, while the other three are still missing.
Bulgarian maritime administration officials, however, still maintain that only three sailors are confirmed dead, with several crewmembers sighted at see on rafts.
The aerial search for survivors, carried out by three helicopters throughout the day, has been called off when darkness descended, but is due to start anew on Friday.
Vanessa is owned by Board Marine, a Burgas-registered company, and was carrying a metal cargo from the Ukrainian port of Berdyansk. Of its crew, 10 were Bulgarian nationals and the other one Ukrainian.
Ship captain Miroslav Dimitrov issued a SOS call at roughly 2.30 p.m., after efforts to right the vessel failed.
A nearby Russian vessel attempted to rescue the sailors at that time, but was unable to do so because of the strong winds.
Vanessa was shipwrecked near the Kerch Strait just 22 nautical miles from the Gulf of Kerch. The area is very tricky during the winter season, with several Russian ships sinking there last month during a storm.
Bulgarian authorities have insisted they should take part in the rescue efforts and their representatives are expected to arrive on location on Friday.