In Sunday's disaster, another four ships sank, and at least six sailors died. The Russian Emergency Situations Ministry said earlier that 2,000 metric tons of fuel oil had spilled into the sea, and that fuel was continuing to flow from the tanker. The four sunken dry-cargo vessels were carrying about 7,000 metric tons of sulphur on board.

Russia's hydrometeorological service said in a statement: "According to estimates, about half of the spilt fuel oil remains on the southwest shore of the Tuzla Promontory [in the Kerch Strait], but about 15% is in the Kerch Strait and will reach the Azov Sea in about 24 hours."

Earlier, experts from the service said that tests of water samples showed that the oil product content in the polluted area was 2.5 milligrams per liter, 50 times above the acceptable level. The service said the slick stretches 20-30 meters into the sea.

Some 9,000 fish and 30,000 birds have been killed by pollution in the strait, earlier reports said.

Rescuers are continuing their search for five missing sailors from the sunken dry freighter Nakhichevan.