The city of San Francisco, California has declared a state of emergency over the deadly coronavirus after health officials warned its spread across the United States was inevitable.

San Francisco Mayor London Breed announced the state of emergency in a statement on Tuesday, saying “the global picture” surrounding the Wuhan coronavirus was “changing rapidly” and city officials “need to step-up preparedness.”

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

There have been not yet been any reported cases of coronavirus in San Francisco. But there are currently over 80, 200 confirmed cases — the majority of those in China — worldwide, and more than 2,700 deaths resulting from the new virus.

The mayor's announcement came as officials with the CDC suggested the virus spreading in the US was not a matter of if, but when.

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Nancy Messonier, director of the CDC's National Centre for Immunization and Respiratory Diseases, said in a statement: "As more and more countries experience community spread, successful containment at our borders becomes harder and harder."

She added: “It’s not a question of if this will happen but when this will happen and how many people in this country will have severe illnesses."

The official also warned that “disruption to everyday life might be severe" as the coronavirus continues to spread.

Health officials also said they want people to start thinking about different kinds of measures they might see if the virus starts spreading, including closing schools and encouraging telework. Those steps were taken during the 2009 H1N1 flu pandemic.

"Part of preparedness is an educated population thinking about the future", said Health and Human Services Secretary Alex Azar.

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He added: "The immediate risk to the general American public remains low, but that has the potential to change quickly."

Mr Azar said that nearly two months after the first alarms were raised, there's no evidence the virus has spread here beyond patients infected overseas and a few close relatives. He credited travel controls and mandatory quarantines, adding that government scientists are working to develop a vaccine and to perfect a test for detecting the virus.

"We cannot hermetically seal off the United States to a virus and we need to be realistic about that," the health chief acknowledged. "We'll have more cases in the United States, and we've been very transparent about that." If it happens, "we'll work to mitigate those."