NORTH BATTLEFORD, Sask. -- Residents of a Saskatchewan city are being told to boil their tap water as a precaution because it might be contaminated.

The Saskatchewan Water Security Agency says the entire city of North Battleford, with a population of about 14,000, is under a precautionary drinking-water advisory.

The advisory says all drinking water -- as well as water used for dishwashing, washing fruit and vegetables and brushing teeth -- must be brought to a roiling boil for one minute.

"A precautionary drinking-water advisory is if there's a potential for a contaminant or something to happen to the system, not necessarily that it has," water agency spokesman Patrick Boyle said Wednesday.

"An emergency boil-water order means that a contaminant has been confirmed and has entered the system, so in this case with North Battleford, it's just a potential at this point."

Dustin MacDonald, the city's acting communications manager, said partially treated water spilled into treated water at one of North Battleford's treatment plants on Tuesday afternoon. That happened for six minutes before alarms went off and the plant was shut down.

"The partially treated water went through the filter system as well as the ultraviolet disinfection system ... the accepted engineering travel time would indicate that the partially treated water was caught within the water treatment plant, but that needs to be scientifically proven that this water did not reach the water distribution system."

Engineers and city staff were doing tests and analyzing data on Wednesday to determine how far the water got, said MacDonald.

"Safety of our residents and visitors is our main concern, and we are working closely with the Saskatchewan Water Security Agency and the Prairie North Regional Health Authority," the city said in a news release late Wednesday afternoon.

"We encourage people to comply with the precautionary drinking water advisory that was issued. Additional water quality testing is underway. At this point it appears it will take two or three days to ensure safe drinking water through further actions and monitoring."

The city said operational error was the cause.

About 8,000 litres of water goes through the system in six minutes -- enough to fill about eight hot tubs, MacDonald said.

More than 7,000 North Battleford residents became sick in 2001 when a parasite called cryptosporidium entered the water supply during routine maintenance of a chemical filter.

Lab tests confirmed 361 cases of illness. No one died.

An inquiry into the outbreak concluded the city systematically failed to recognize its responsibilities for the water supply.

An inquiry report also criticized the province's Environment Department as being an "inadequate and ineffective" regulator. The city's water treatment plant had not been inspected by the province for 10 years.

At least 800 people who sued over the outbreak reached out-of-court settlements paid by the city and the Saskatchewan government.