It wasn't immediately clear what was tainting the river, the country's second longest.

Residents were alarmed to see a sewage pipe pouring red water into the river on Sunday between 3pm and 6pm, according to media reports.

Environmental protection officials took samples and were trying to determine whether the sewage was toxic.

A photo in a local paper showed a resident by a stretch of the river - a drinking water source for millions - that was rose-colored instead of the usual milky brown. Other photos showed patches of bright red and pink.

An official from the Yellow River Water Resource Committee in Lanzhou confirmed the pollution. He said the committee was still analyzing the sample and had not determined what caused it.

Kang Mingke, an official with the city's environment protection bureau, said test results would determine what the red discharge was and whether it was detrimental to the river's ecology. He said yesterday morning the test results were not yet available.

Kang said the red water could be from central heating systems, as there are no chemical plants located nearby.