Rescuers are searching for 20 sailors missing after a powerful storm hit the Azov and Black Seas on Sunday.
Russian officials say four ships have sunk, including an oil tanker, and four others could break up. The bodies of three sailors have been found.
Up to 2,000 metric tons of fuel oil have leaked from the tanker in the Kerch Strait, which links the two seas.
A big clean-up operation is under way, amid fears of an environmental disaster in the region.
At least two other ships were carrying potentially hazardous cargos when they sank, including nearly 6,000 tons of sulphur.
A new storm warning in the region was issued again on Monday.
The Russian oil tanker Volganeft-139 came apart after it was smashed by 108km/h (67 mph) winds and 5m (16ft) waves in the narrow Kerch Strait between Russia and Ukraine.
The tanker's 13 crew were rescued after several hours. So far, 35 sailors from the sunk vessels have been plucked to safety.
The bodies of three sailors from the stricken ships were found on Monday morning, but five others are still missing, Russian officials say.
Some 300km (187 miles) to the south-west, rescue teams are continuing their search for another 15 sailors missing after a a scrap metal ship went down near the Ukrainian port of Sevastopol.
A second oil tanker in the region is being monitored closely because its hull has developed cracks.
Yet more ships have run aground or slipped anchor and are drifting at the mercy of the storm.
Russian prosecutors say they are investigating whether the ships' captains ignored warnings of the approaching storm.
'Sinking to seabed'
The Volganeft-139 was at anchor when its stern tore apart in Ukrainian waters on the busy waterway dividing that country and Russia, officials said.
A regional prosecutor told local media the tanker was designed in the Soviet era to transport oil on rivers and was not meant to withstand heavy storms.
Russian environmentalist Vladimir Slivyak said the tanker accident was a "very serious environmental disaster".
The heavy oil was sinking to the seabed and could take years to clean up, he said.
But the oil spill is small by comparison with the Prestige disaster off Spain five years ago.
Severe habitat damage was caused to beaches in Spain, France and Portugal when a tanker leaked 64,000 tons of fuel oil in November 2002.
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