WARSAW (AP) - Polish authorities on Thursday (Aug 29) were warning residents in cities along the Vistula river that runs into the Baltic Sea of a "crisis" situation after Warsaw's new sewage collection plant malfunctioned.

The health minister and local authorities said they have been closely monitoring the levels of contamination north of the Polish capital, including by nitrogen, since the problem began late on Tuesday, but wouldn't disclose the results of the tests.

The sewage is being discharged at about 3,000 litres a second at Warsaw's northern edge and goes north without affecting the city's waters, the authorities said.

"There is no reason for panic and there is no threat to the health of Warsaw residents," Warsaw Mayor Rafal Trzaskowski said.

Other officials were speaking in the central city of Plock, about 110km north of Warsaw, which was bracing itself for a wave of contaminated water.

Health Minister Lukasz Szumowski advised people not to fish or bathe in the Vistula and to boil water before use, even for brushing teeth.

Experts were working to fix the malfunction at the sewage plant, which will take longer than three days, according to Trzaskowski.

It was not immediately clear what caused the sewage collection system, including an emergency backup, to fail.