The death of tonnes of fish in a river in Europe is shrouded in mystery, amid speculation from Poland’s prime minister that “huge amounts of chemical waste” were probably dumped intentionally into the water.

There are fears of a major environmental catastrophe in the River Oder, which runs from the Czech Republic to the border between Poland and Germany before flowing into the Baltic Sea.

Tonnes of dead fish have been seen floating or washed ashore on the Oder’s banks over the past two weeks and the issue has erupted into a major scandal.

Polish leader Mateusz Morawiecki has vowed to hold those responsible to account and his government has posted a reward of one million zlotys (£180,000) for information.

“Huge amounts of chemical waste were probably dumped in the River Oder with full awareness of the risks and consequences,” he said in a video on Facebook.

“We will not let this matter go. We will not rest until the guilty are severely punished.”

Lab tests following the mass die off of fish detected high levels of salinity but no mercury poisoning, meaning the cause of death remains unclear.

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Some German media had suggested the river have been be poisoned with mercury.

“The extent of the fish die-off is shocking. This is a blow to the Oder as a waterway of great ecological value, from which it will presumably not recover for a long time,” said Alex Vogel, the environment minister for Germany’s Brandenburg state.

Poland’s national water management authority said on Thursday that 10 tonnes of dead fish had been removed from the river.

Hundreds of volunteers were working to help collect dead fish along the German side.

Authorities in the northeastern German state of Mecklenburg-Western Pomerania have warned people not to fish or use water from the Szczecin lagoon.

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