Residents of Canada's sixth-largest city were advised to keep boiling tap water before using it late Wednesday, even though latest tests showed no traces of harmful bacteria, officials said.

Winnipeg health officials want to review secondary on Thursday before deciding whether to lift the boil-water advisory on the city's 700,000 residents—the first in the city's history.

No infections have been reported at city hospitals since the E. coli bacteria was detected in the water supply on Monday.

Symptoms include stomach pains and generalized vomiting.

Residents were warned to boil before drinking it, using it to prepare food or brushing their teeth.

They also were urged to help toddlers or the elderly bathe to keep water from getting into their mouths.

The city said it issued the boil water advisory as a precaution after what it said were "atypical test results" showing E. coli in six of 39 samples of city water, despite chlorine levels that surpassed requirements.

"Accidental contamination" of the water could explain the positive tests, it said.