It was reported to be one of the worst environmental disasters in years in the region around the Black Sea.

Waves as high as 5m in the Kerch Strait, a busy narrow waterway running between the Azov and Black Seas, tore apart the prow and the stern of the oil tanker belonging to Russian oil firm Volganeft.

It was carrying fuel oil from the southern Russian city of Samara on the Volga River to an oil terminal in Ukraine, agency reports quoted a Russian official as saying.

The emergency authorities said the tanker's 13 crew members had been rescued.

The likely effects of the spill were not immediately clear - a spill over 700 tonnes is considered large and can kill thousands of marine animals and destroy habitats, as well as affect the fishing and tourist industries.

Several other ships were also in trouble yesterday in the Kerch Strait, which is between Russia and Ukraine.

Worsening weather in the area sank two cargo ships, each carrying some 2,000 tonnes of sulphur.

Twelve crew members from these ships had been rescued, but the fate of eight others was unclear.

As wind speeds reached 108kmh, several other ships were reported damaged in and around Kavkaz, the Russian commercial port some 1,200km south of Moscow.

A total of 42 vessels had been evacuated from the port because of the risky weather conditions, but 17 others were still there, Russian news agencies reported, citing an Emergency Situations Ministry spokesman.

A Georgian freighter and a Turkish one were also stranded off the nearby Russian Black Sea port of Novorossiisk, RIA news agency quoted port chief Vladimir Yerygin as saying.

Russian television reported that the spill from the Volganeft-139 tanker, which was carrying 4,000 tonnes of fuel oil, was continuing and that the accident happened in Ukrainian waters.

Russia and Ukraine have set up a joint crisis centre to deal with the situation.

Ukraine's maritime agency said all the country's ports had been put on a heightened state of alert.

The Volganeft-139 was stranded about 5km from shore.

Stormy weather was preventing emergency workers from collecting the spilt oil, said the authorities.

Mr Oleg Mitvol, head of Russia's environmental monitoring agency Rosprirodnadzor, called the oil spill a 'very serious environmental disaster'.

He said: 'Fuel oil is a heavy substance and it is now sinking to the seabed.'

Mr Vladimir Slivyak, head of Russian environmental group Ekozashchita, or Ecodefence, called it 'a major ecological catastrophe'.

He was quoted by Interfax news agency as saying: 'The pollution that has taken place will have to be cleaned up for a long time to come and the consequences will be felt for a year or even more.'

Mr Maxim Stepanenko, a regional prosecutor in Russia, told Vesti 24 television that the oil tanker - designed during Soviet times to transport oil on rivers - was not built to withstand a fierce storm.

Mr Mitvol said that while the spilt sulphur from the sunken ships did not present an environmental danger, the two freighters could also leak fuel oil from their tanks, adding to the pollution.

REUTERS, ASSOCIATED PRESS, AGENCE FRANCE-PRESSE,