Russia's environmental watchdog, which is investigating unexplained toxic pollution off the country's far eastern coast, has said it found significantly higher levels of pollutants in nearby rivers than in the affected waters themselves.

Greenpeace warned last week of an ecological disaster in waters off the Russian region of Kamchatka, a volcanic peninsula on the Pacific, where large numbers of sea creatures died, their carcasses washing up on shore.

Russia opened a criminal investigation on Wednesday to establish the cause of the pollution. Conservation group World Wildlife Fund (WWF) has said it was very likely caused by a highly toxic soluble substance.

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Watchdog Rosprirodnadzor said in a report on Thursday it found high levels of phosphates, iron and phenol in rivers that enter the Avacha bay in the Far East, several times more than in the waters off the coast.

Greenpeace said that the tests did not include enough data, that the cause of the pollution remained unknown and that the official inquiry was taking a worrying amount of time.

"The results that have been received are not enough to show the full picture of what has happened," Vladimir Chuprov, Greenpeace Russia project director, said in a statement, adding that an analysis of the water's heavy metal content was still being conducted.

Laboratory analyses of samples taken from the animals that died were also lacking, Chuprov said.

The Investigative Committee, a Russian equivalent of the US FBI, has said that dead marine life had washed up on the shore from September 1 to October 3, and that the sea water had been found to contain oil components including phenol and had changed colour. 

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