Russian rescue helicopters and ships searched for five missing seamen on Monday after a storm in the northern mouth of the Black Sea, while oil spilt from a sunken tanker coated birds in a black sludge.

Rescue officials said three people died in the storm that struck the narrow Kerch Strait between the Black Sea and the Azov Sea on Sunday, sinking a small oil tanker and at least four freighters and leaving other ships stranded on the shoreline.

Birds seeking shelter on the shore near the centre of the storm were covered in a treacly mixture of oil and seaweed -- the first evidence of what one Russian official called an environmental disaster.

At Novorossiisk, Russia's No. 2 port for exports of oil and oil products, officials had ordered tankers not to dock because a new storm was on its way. Reloading of oil from damaged tankers was also suspended in the Kerch Strait later on Monday.

The worsening weather was also hampering rescue operations, said Anatoly Yanchuk, a rescue department chief at Russia's Transport Ministry.

We will continue efforts to find those five missing, but the chances of finding them are now smaller, he told reporters in the port of Kavkaz overlooking the strait. The weather is worsening and the number of rescue vessels has been cut.

OIL-COVERED BIRDS

The oil spill came from the Volgoneft-139, a small Russian oil tanker which broke in two during the storm when it was off the Ukrainian port of Kerch.

Officials said it had spilled at least 1,300 tons of fuel oil. In cold weather, the thick, treacly substance may sink to the seabed instead of dispersing, making the clean-up harder.

The tanker was carrying 4,000 tons of fuel oil in total when it was hit by the storm. A spill of over 700 tons is considered large, but the biggest ones run into the tens or even hundreds of thousands.

At the coastal settlement of Ilyich, halfway between Kavkaz and Novorossiisk, about 100 workers were on the beach using shovels and a bulldozer to scrape globules of oil off the sand.

This oil came in last night, along a 13 km (8 miles) stretch, said Alexander Mikhalkov, a clean-up crew foreman.

A flock of about 1,000 rails, a species of wetland bird, were huddled on the beach, unable to fly because their feathers were coated with oil. Some were unable to stand.

The spill raised questions about maritime safety in the Kerch Strait, a busy waterway which separates Ukraine's Crimea peninsula and southern Russia.

Ukrainian Prime Minister Viktor Yanukovich said that at the moment the slick was moving away from Ukraine, but measures should be taken to prevent future disasters.

In the Borsphorous Straits, it's not possible to use tankers which have no double hulls. How is the Kerch strait different? It isn't, he said at a news briefing in Kiev.

Environmental campaign group Greenpeace said the oil spill revealed the shortcomings of shipping safety in the region.

In Russia we do not have one hundred percent of our ships maintained in a suitable condition as is the practice in the West, Alexei Kiselyov, coordinator of Greenpeace Russia's anti-pollution campaigns, told Reuters.

In the last few days we have seen a very clear demonstration of that.

Oleg Mitvol, deputy head of Russia's environment agency Rosprirodnadzor on Sunday called the spill a very serious environmental disaster, he said on Sunday.

(Additional reporting by Vera Kalian, Dmitry Madorsky and Natalya Zinets, writing by Christian Lowe and Dmitry Solovyov; editing by Sami Aboudi)

Russia Oil Spill