Coronavirus risk in U.S. still 'low': CDC

McQuiston added that the risk of the outbreak in the United State was 'low'.

On Wednesday another group of nearly 350 Americans airlifted from the epicenter of China's fast-spreading coronavirus outbreak were placed under quarantine at two military bases in California on Wednesday, as the number of confirmed U.S. cases of the disease rose to 12.

The travelers aboard two State Department-chartered cargo jets were met by personnel from the Centers for Disease Control and Prevention (CDC) for screening, and are to remain under mandatory quarantine for 14 days, the incubation period of the virus.

Five of the passengers arrived exhibiting possible symptoms, two of them children, and were immediately hospitalized, along with the mother of one of the youngsters, the CDC said.