SAN FRANCISCO, Feb. 26 (Xinhua) -- San Francisco Mayor London Breed on Tuesday declared a state of emergency for the city to heighten preparedness for the possible spread of the novel coronavirus as more cases of infections were reported worldwide.

"Although there are still zero confirmed cases in San Francisco residents, the global picture is changing rapidly, and we need to step-up preparedness," Breed said.

"We see the virus spreading in new parts of the world every day, and we are taking the necessary steps to protect San Franciscans from harm," she added.

The San Francisco emergency declaration is effective immediately for seven days, which will be voted on by the city's Board of Supervisors on March 3.

The local emergency measure came after the U.S. Centers for Disease Control and Prevention (CDC) warned earlier in the day of a wider circulation of the virus in the country.

"Now is the time for U.S. businesses, hospitals, and communities to begin preparing for the possible spread of #COVID19 (coronavirus disease 2019)," the CDC tweeted Tuesday.

The agency also said that the number of cases in the United States has risen to 57, with four more patients who were on a cruise ship.

San Francisco officials have also urged schools and businesses to make plans for any contingencies, including possible closures.

San Francisco has no home-grown cases of coronavirus infections, but it has treated three infected patients from other places in its hospitals.

It is the most recent Bay Area city to declare a local emergency due to coronavirus concerns. The previous city was Santa Clara County, where two confirmed cases were reported.  ■