Federal Environment Minister Steffi Lemke insists on stopping the development of the Oder.
"The extent of the damage to the valuable ecosystem of the Oder is not yet foreseeable," said Lemke of the German Press Agency in front of the German-Polish Environmental Council in Bad Saarow in Brandenburg.
The Green politician and her Polish colleague Anna Moskwa discuss the cause and consequences of the environmental disaster.
"Further negative influences must be avoided," said Lemke.
"Against this background, in my opinion, the expansion of the Grenzoder is extremely problematic and must now be stopped." The cause of the fish kill has not yet been found.
A joint group of experts has been set up to investigate the causes.
Severely damaged ecosystem
It is also about taking steps "how we can restore the badly damaged ecosystem of the Oder together with Poland".
The cross-border processes in such incidents must also be improved so that such a catastrophic extent can be prevented in the future, said Lemke.
Environmental and nature conservation organizations are pushing for an action plan to save the German-Polish border river.
According to information from the Brandenburg Environment Ministry, the first results of a bilateral group of experts on fish kills will be presented on Monday.
Lemke said in an interview that she expects more serious damage to the Oder than just the fish kill.
The environmental organization WWF Germany considers the Oder to be poisoned over a large area.
Masses of dead fish were discovered in the German-Polish border river in August.
The exact reason for this is so far unclear.
Experts assume that high salinity in the river is a major reason, combined with low water, high temperatures and a toxic species of algae.
By Saturday a week ago, around 200 tons of fish carcasses had been collected in Poland and Germany.
Environmental groups are calling for an Oder rescue plan.
The fish kill caused upset in German-Polish relations.
Germany criticized Poland's information policy, while Poland's government spoke of "false news" from Germany.
The expansion of the Oder planned by Poland should also be an issue for the environmental council.
Other topics of the meeting include the search for a nuclear waste repository in Germany and the Polish use of nuclear power.