The Bulgarian authorities say army and civil defence units have been deployed to prevent an oil slick in the Danube river from reaching the country's only nuclear plant in Kozloduy, two-hundred kilometers north of the capital, Sofia.

They are working on defences to stop the oil from reaching the channel which leads to the cooling sytem of the plant.

The slick is fifty-five kilometres long and was expected to reach the plant on Thursday morning.

It is not known what caused the oil spill.