Polish firefighters said Tuesday they had recovered 100 tonnes of dead fish from the Oder river running through Germany and Poland, deepening concerns of an environmental disaster.

"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.

Water samples have 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.