Poland deployed soldiers to help clean the Oder River after an environmental disaster caused tons of fish to die.

A fishermen's association in Zielona Góra, a city in western Poland, said on Friday that it would suspend fishing in the river due to pollution.

Defense Minister Mariusz Błaszczak announced on Thursday that soldiers and reservists were being deployed to help remove pollutants from the river known as the Oder in German, and Odra in Polish and Czech, which flows north for hundreds of miles from the Oder Mountains in the Czech Republic and empties into the Baltic Sea.

Polish Prime Minister said on Friday that "huge amounts of chemical waste" may have been deliberately dumped into the Oder River, which runs along the border with Germany, causing severe environmental damage so severe that the river will take years to recover.

Tons of dead fish were seen floating or drifting ashore along the banks of the Oder River over the past two weeks, but the issue did not become a major scandal until late this week.

Prime Minister Mateusz Morawiecki, whose government is under pressure for its handling of what appears to be a major environmental disaster, pledged that Polish authorities will hold the perpetrators accountable.

He said in a Facebook video, "Huge amounts of chemical waste may have been dumped into the Oder River with full awareness of the risks and consequences." He added, "We will not let this go. We will not rest until the guilty are severely punished."

Media reported that the poison is mercury, although this has not been officially confirmed.

Investigations are currently underway to determine the cause of the fish deaths, and huge numbers of dead fish were first observed near the town of Oława in southwestern Poland in late July, along with dead animals such as beavers.

Przemysław Daka, head of the Polish Water Authority, the national water management agency, said on Thursday that 10 tons of dead fish had been removed from the river.

He said at a press conference near the river, "This indicates that we are dealing with a huge and shameful environmental disaster," as officials faced angry residents.

At the same time, German officials complained that Poland failed to respect an international treaty by not promptly notifying them of potential pollution in the river. A boat captain first alerted German authorities about the dead fish on August 9.

Christopher Stoltenberg, spokesperson for the German Federal Ministry for the Environment, told reporters in Berlin, "We know that the chain of reports intended for such cases did not work."

Stoltenberg said that German authorities are in contact with their Polish counterparts to obtain more information about the situation and to offer any required assistance.