According to the Polish government, the death of fish in the Oder is not due to heavy metals.

This would have resulted in further analyzes of dead fish by the State Veterinary Institute, Environment Minister Anna Moskwa wrote on Twitter on Saturday evening.

The government in Warsaw had previously ruled out elevated mercury levels as the cause.

However, the analyzes indicated increased salt levels in the water and thus agreed with the findings of the German authorities, Moskwa told the PAP news agency.

"The high salinity of the Oder may have activated other toxic substances in the water or in the bottom sediment.

Toxicology testing of the fish will help identify any contaminants that may have contributed to the deaths of the animals.”

According to information from Brandenburg's Environment Minister Axel Vogel (Greens), the Oder has "very much increased salt loads".

The term salt loads refers to salts dissolved in the water.

Poland's government suspects that the river was poisoned with chemical waste.

The Polish police have offered a reward of the equivalent of 210,000 euros for the investigation.

According to the Ministry of the Interior in Warsaw, 2,000 police officers, more than 300 firefighters and 200 soldiers are currently deployed on the banks of the Oder.

They help rescue dead fish and warn citizens to avoid contact with the water.