Residents of the country's fourth-largest city were told to boil water Sunday after pump-driven water system pressure dropped because of an electrical outage, triggering concerns of possible contamination.

The boil notice essentially covers Houston's 2.3 million residents. Bottled water is an acceptable substitute, public works officials said in a statement.

The agency advised the public to avoid drinking water without boiling it first, to bring all water to a boil for at least two minutes before using it and to let the water cool before using it. People without power are encouraged to use bottled water.

Houston Mayor Sylvester Turner said the city believes the water is safe, but they had to issue the boil water notice due to regulatory requirements.

"Water samples will subsequently follow and hopefully we will get the all clear from TCEQ," Turner tweeted early Monday morning. "The City has to wait 24 hours from that point before the boil water notice is suspended. The earliest would be tomorrow night or very early Tuesday morning."

The Houston Independent School District announced its schools, offices and facilities would be closed on Monday in response to the boil water notice.

A power outage Sunday at the East Water Purification Plant caused pressure to drop below 20 psi, or pounds per square inch, the state threshold that triggers boil water notices, Houston Public Works said.

Water system pressure can use the weight of liquid to occupy cracks and crevices that might otherwise be exposed to outside incursions, such as urban runoff.