California has only 200 test kits, but has appealed for testing protocols to be expanded to include Americans who may catch the virus as it spreads through US communities, Newsom said.

Newsom said the US Centres for Disease Control and Prevention (CDC) has assured the state that testing protocols will be advanced and access to test kits will expand “exponentially” in the coming days.

California on Wednesday reported a coronavirus case of unknown origin, making it potentially the first incident of the virus spreading within US communities.

“It is not surprising that we have seen these mini outbreaks occur around the world,” said Dr. Jeffrey Klausner, an epidemiology professor at the University of California, Los Angeles Fielding School of Public Health. “We know the virus is easily spread.”

The female patient, who lives in northern California’s Solano County, was not initially tested for the coronavirus because she did not meet criteria laid out by the CDC, according to a statement by UC Davis Medical Centre in Sacramento, where she is being treated.

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Dr. Sonia Angell, California’s state public health officer, said local, state and federal health investigators were contacting and isolating individuals who may have been exposed to the patient.

Solano County is the home of Travis Air Force Base, one of the US military bases in California that have acted as quarantine centres for US citizens repatriated from Wuhan, China, the epicentre of the current coronavirus outbreak.

UC Davis said no one on its main campus has tested positive for the virus, but “out of an abundance of caution,” three campus residents are currently in isolation. One of them, who is showing symptoms of illness, has been tested for the virus, but the results are not yet in.

The CDC’s protocol had called for coronavirus testing in individuals with a fever or respiratory illness who have a travel history or contact that could have led to exposure to the virus.

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CDC Director Robert Redfield told a US congressional hearing on Thursday that more test kits were being sent to California and the agency now backs expanded testing.

“When a clinician or a public health individual suspects coronavirus, then we should be able to get a test for coronavirus, so that is the current guidance that went out today,” he said.

Newsom said the CDC has pledged to send more personnel to California to track the potential spread of the virus.

With new infections reported around the world now surpassing those in mainland China, the World Health Organisation’s (WHO) director general, Tedros Adhanom Ghebreyesus, said even rich nations should prepare.

“No country should assume it will not get cases, that would be a fatal mistake, quite literally,” Tedros said, pointing to Italy, where 17 people have died in Europe’s worst outbreak.

Trump said the number of infections could increase before getting better, or the virus could go away.

“We have really been able to keep the risks to Americans low right now,” Health and Human Services Secretary Alex Azar said.

“That could change. That is why it’s important for all of us to prepare.”

Also on Thursday, a lawyer representing a whistle-blower accused the US Department of Health and Human Services of a “failure to protect its employees” responsible for responding to coronavirus.

Additional reporting by Bloomberg