Polish firemen pull tonnes of dead fish from Oder river

"We'd never had an operation of this scope on a river before," said Monika Nowakowska-Drynda from the national firefighter press office.

She confirmed that around 100 tonnes (220,500 pounds) of dead fish had been recovered since Friday.

The cause of death is uncertain, though officials believe the fish are likely to have been poisoned.

"Probably enormous quantities of chemical waste was dumped into the river in full knowledge of the risk and consequences," Polish Prime Minister Mateusz Morawiecki said last week.

But Climate and Environment Minister Anna Moskwa said on Tuesday that "none of the samples tested so far has shown the presence of toxic substances".

She said the government was also looking into possible natural causes and in particular higher concentrations of pollutants and salinity as a result of lower water levels and high temperatures.

A third hypothesis being examined is that industrial waste water with a high chlorine content was poured into the river, she said.

Water samples have also been sent to laboratories in the Czech Republic, the Netherlands and Britain in the hopes of finding the cause.

The first reports of mass fish deaths were made by Polish locals and anglers as early as July 28.

German officials have accused Polish authorities of failing to inform them about the deaths, and were taken by surprise when the wave of lifeless fish came floating into view.

In Poland, the government has also come under heavy criticism for failing to take swift action.

The Oder has over the last years been known as a relatively clean river, and 40 domestic species of fish are found in the waterway.

But now, dead fish -- some reaching up to 40 centimetres (16 inches) -- can be seen across the river.

Nowakowska-Drynda said more than 500 firefighters have been recovering the dead fish in Poland with the help of dams, boats, quad bikes and even a drone.