Tests following a mass die-off of fish in Central Europe’s Oder River have detected high levels of salinity but no mercury in the water, Poland’s environment minister said.

Anna Moskwa said analyses of river samples taken in both Poland and Germany revealed the elevated salt levels.

Comprehensive toxicology studies are still under way in Poland, the minister added.

Writing on Twitter, Ms Moskwa said test results transmitted from Germany had so far not shown a high presence of mercury.

The Oder River runs from the Czech Republic and along the border between Poland and Germany before flowing into the Baltic Sea.

Some German media had reported that the river could be poisoned with mercury.

Polish Prime Minister Mateusz Morawiecki said on Friday that “huge amounts of chemical waste” were probably dumped intentionally into his country’s second-longest river, causing environmental damage so severe it would take years for the waterway to recover.

On Saturday, Mr Morawiecki vowed to do everything possible to limit the environmental devastation.

Poland’s interior minister said a reward of one million zlotys (£164,000) would be paid to anyone who helps track down those responsible for polluting the river.

Authorities in the north-eastern German state of Mecklenburg-Western Pomerania warned people not to fish or use water from the Szczecin lagoon as the river’s contaminated water was expected to reach the estuary area on Saturday evening.

Alex Vogel, the environment minister for Germany’s Brandenburg state, along which the river runs, said: “The extent of the fish die-off is shocking.

“This is a blow to the Oder as a waterway of great ecological value, from which it will presumably not recover for a long time.”

The head of Poland’s national water management authority said on Thursday that 10 tonnes of dead fish had been removed from the river. Hundreds of volunteers are working to help collect dead fish along the German side.

German laboratories said they detected “atypical” levels of “salts” that could be linked to the die-offs, but would not explain them on their own.

Mr Morawiecki acknowledged that some public officials had been “sluggish” in reacting after huge numbers of dead fish were first seen floating and washing ashore.

Two Polish officials were dismissed for what Mr Morawiecki described as tardiness in their response.

“If I come to the conclusion that there was a serious breach of duties, further consequences will be drawn,” the prime minister said.

“For me, however, the most important thing is to deal with this ecological disaster as soon as possible, because nature is our common heritage. It is a national good.”

His comments were echoed by Schwedt mayor Annekathrin Hoppe, whose German town is located next to Lower Oder Valley National Park.

She called the contamination of the river “an environmental catastrophe of unprecedented scale” for the region.