Polish Prime Minister Mateusz Morawiecki has admitted his country was slow to react to an ecological disaster unfolding in the Oder River, firing two top officials.

Thousands of dead fish have been found along the river running between Poland and Germany, with both countries warning of an environmental disaster.

The Polish government has stressed the need to find and punish those responsible for the disaster.

Morawiecki fired Przemysław Daca, head of Poland's national water management authority, and Michał Mistrzak, chief of the general environmental inspectorate.

According to Deputy Climate and Environment Minister Jacek Ozdoba, they are most likely dealing with a crime where a substance was introduced into the water.

The police chief has set a 1 million zloty ($220,200) reward for help in finding the perpetrators.

The Oder river flows to Poland from the Czech Republic, and then along a long stretch is a border between Poland and Germany.

On Sunday, near Czelin in Western Pomerania, barrels with a suspicious substance were found. It is not known yet what is in them, but reportedly a petroleum substance.

In Germany, Brandenburg Environment Minister Axel Vogel has said the reason for such a massive contamination of the river's ecosystem could be the high salt concentration in the water.

Besides dead fish, dead birds and beavers have also been found in the Oder and its vicinity for several days.