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 and the Polish government 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 potential 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.

Dead fish can now be found some 300 kilometers north of Olawa where the Oder flows along the German border, Ewa Drewniak, a biologist working with opposition group Civic Coalition told Reuters on Thursday, accusing the government of not responding quickly enough.

"Dead fish have been flowing in the Oder for the past two weeks and people have not been informed about it, I've seen scores of people bathing in the river a week and a half ago, they were not aware of the danger, this is scandalous," she said.

Regional environmental protection authorities in Wroclaw said that Oder water samples taken on July 28 showed an 80% 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 a strong oxidizing agent might have gotten into the water causing the oxygen levels to rise sharply, which can harm fish. He added 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 as long as the cause of the fish deaths was unclear.

"The reports from the Oder are simply terrible," Michael Kellner, a senior politician of Germany's Greens, said on Twitter.

(Reuters)

Dead fish in River Oder on Polish/German border spur contamination probe