Huge amounts of chemical waste were probably dumped intentionally into the Oder River, which runs along the border with Germany, causing environmental damage so severe it will take the river years to recover, Poland’s prime minister said on Friday.

Tonnes of dead fish have been seen floating or washed ashore on the Oder’s banks over the past two weeks, but the issue only erupted into a major scandal late this week.

Prime minister Mateusz Morawiecki, whose government is under pressure for its handling of what appears to be a major environmental catastrophe, said Polish authorities would hold the perpetrators to account.

“Huge amounts of chemical waste were probably dumped in the Oder River 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.”

German media have reported that the poison is mercury, although this has not been officially confirmed.

Investigations are under way to determine the cause of the mass fish deaths. Huge numbers of dead fish were first spotted near the south-western Polish town of Olawa in late July, along with dead animals such as beavers.

Przemyslaw Daca, head of Polish Waters, the national water management authority, said Thursday that 10 tonnes of dead fish have been removed from the river.

“This shows that we are dealing with a gigantic and outrageous ecological catastrophe,” he said at a news conference near the river where officials faced angry residents.

Meanwhile, German officials complained that Poland failed to honour an international treaty by not notifying them immediately about the possible contamination of the river.

A boat captain first alerted German authorities about the dead fish on August 9.

“We know that the chain of reporting that’s envisaged for such cases didn’t work,” Christopher Stolzenberg, a spokesman for Germany’s federal environment ministry, told reporters in Berlin.

Mr Stolzenberg said German authorities were in contact with their Polish counterparts to get further information about the situation and to provide any assistance requested.

Poland has deployed soldiers to help clean up the Oder and an association of fishermen in Zielona Gora, in western Poland, said Friday it was suspending fishing in the river due to the contamination.

According to Mr Morawiecki, the scale of the pollution is so large that it may take years for the river ecosystem to recover.

Defence minister Mariusz Blaszczak said on Thursday that soldiers and reservists were being deployed to help remove pollutants from the river, which is known as the Oder in German and the Odra in Polish and Czech.

It flows north for hundreds of miles from the Oder Mountains of Czechia and empties into the Baltic Sea.