Tonnes of dead fish have been hauled out of the River Oder which flows along part of Poland's border with Germany, as water officials warned people from entering the river. The Polish government has pledged to investigate possible contamination.
Firefighters and volunteers have removed about five tonnes of dead fish from the town of Olawa in southwest Poland, officials at State Water Holding, which manages Polish national waters, said on Wednesday.
Environmental protection authorities said they had notified prosecutors about possible contamination of the river, the country's second longest.
Polish government officials are due to visit Olawa later on Thursday. Prime Minister Mateusz Morawiecki said any contamination was scandalous and vowed to punish those responsible.
Regional environmental protection authorities in Wrocław said that Oder water samples taken on July 28 showed an 80 percent probability that they contained methylene, a toxic substance, although this wasn't present in samples taken after August 1.
Low river levels due to the drought in Europe might have aggravated possible contamination, the head of State Water Holding, Przemyslaw Daca said on Wednesday.
He added they suspected that a strong oxidising agent might have got into the water causing the oxygen levels to rise sharply, which can harm fish. He went on to say that there was no reason for panic and the situation was improving.
Meanwhile, several German districts bordering Poland warned locals to avoid the river water and not to eat fish from the Oder whilst the cause of the fish deaths remains unclear.