The government on Monday rejected a claim from Poland that Germany had spread fake news following the deaths of large numbers of fish in the Oder River.

According to the government, there may be multiple reasons for the die-offs along the waterway, which forms a large stretch of Germany's border with its eastern neighbor.

What is behind the accusations of 'fake news'?

Environment Ministry spokesman Andreas Kübler said the government was "surprised and saddened by Warsaw's suggestion that Germany was spreading "fake news."

"More fake news is being spread in Germany," Polish Environment Minister Anna Moskwa wrote on Twitter on Saturday, in reference to reports that a laboratory had detected excessive pesticide levels in the water.

Kübler said the cause of the fish die-offs was complicated and added that Germany had never suggested that pesticides alone were to blame.

"It really seems to be a chemical cocktail. According to our findings so far, none of these substances could have led to the fish die-off on its own," he said, adding that the die-offs could be a "multicausal event."

"No one in Germany has ever claimed that the pesticides were the sole cause of the fish deaths," Kübler said. "It is regrettable that the laboratory results have now been interpreted by the Polish Ministry of the Environment as an apportionment of blame."

Kübler said several organic and inorganic substances could be responsible.

In its assessment, the Polish Environment Ministry said pesticide levels were below the threshold that could have an effect of fish and other species.

More than 100 metric tons (220,000 pounds) of dead fish have been recovered since July from the river. The catastrophe has soured relations, with Berlin accusing Warsaw of failing to communicate the disaster and act quickly enough. The first fish die-offs were noticed by fishermen on the Polish side.

Both agree on toxic algae

Kübler also told reporters that the fish deaths could have — as suggested by Poland — been partially caused by toxic algae in the water.

"A mass development of toxic brackish water algae could have contributed to the fish deaths," he said.

Kübler added, however, that the formation of such algae is "not a purely natural phenomenon" and "does not occur to this extent ... under natural conditions."

He said the algae had likely developed because of high salinity levels in the river, which "would not normally exist in the Oder and which can only exist through industrial discharge," he said.

Polish Prime Minister Mateusz Morawiecki has suggested that chemical waste may have been responsible.

Environment Minister Moskwa said last week, however, that "none of the samples tested so far has shown the presence of toxic substances."

Meanwhile, German Environment Minister Steffi Lemke said the disaster underlined the fragility of the Oder as an ecosystem. She called for an end to any planned deepening or widening of the river for navigation.

"Healthy and intact rivers help to stabilize the water balance and can mitigate droughts," she said.

Companies and ministries from the Czech Republic, Germany and Poland have all in the past expressed an interest in expanding the Oder for river traffic.

rc/rt (dpa, Reuters, AP)