Oleg Mitvol, deputy head of state environment watchdog Rosprirodnadzor, said the oil had polluted a 50-km (30-mile) long stretch of Black Sea coastline and rescue workers would have to remove 10,000 tonnes of oily sludge from the shore.

Mitvol said around 1,500 tonnes of fuel oil was still afloat in the water, killing thousands of birds and fish.

"Unfortunately, not only fish are affected, but sea mammals as well -- we have documented cases of deaths of dolphins," he told a news conference.

"The fact that dolphins and birds listed in Russia's Red Book (of endangered species) are dying is very sad indeed ."

Environmentalists say the Black Sea dolphin is on the verge of extinction. The UN Environment Programme has declared 2007 the Year of the Dolphin.

Mitvol said environmental experts from Belgium, Brazil and the United States would arrive in the area in the next few days.

"Volunteers from all across Russia are heading there, as well as groups from Greenpeace and WWF," he said.

OLD ROWS WORSEN CRISIS

The Kerch Strait separates the port of Kerch on Ukraine's Crimea Peninsula from Russia.

Mitvol hit out at the head of the port, saying he had prevented storm-battered ships from using a channel alongside the port to reach calmer waters.

Mitvol also said that Ukrainian environmental inspectors had tried on Wednesday to stop Russian clean-up workers who were pumping the remaining oil out of the tanks of the partially-sunken tanker.

He said Ukrainian authorities were resisting a Russian proposal to contain the oil spill by building a dam across one of the channels in the Kerch Strait.

The area has been the subject of a territorial dispute between Russia and Ukraine. The two have had fraught relations since a 2004 bloodless revolution brought a Western-leaning president to power in Kiev.

Russia's attempts to build a breakwater stretching from its southern Krasnodar Region to Tuzla sparked a fierce territorial dispute between Moscow and Kiev four years ago. The row has not yet been resolved.