The Trump administration, while insisting the risk to Americans from coronavirus is low, had declared a public health emergency on Friday and announced the extraordinary step of barring entry to the United States of foreign nationals who have travelled to China.

In addition, starting on Sunday, US citizens who have travelled within the past two weeks to China's Hubei Province - epicentre of the coronavirus epidemic - will be subject to a mandatory quarantine of 14 days: the incubation period of the virus, officials said.

The emergency measures were unveiled by US Health and Human Services secretary Alex Azar at a White House briefing, shortly before the Centers for Disease Control and Prevention (CDC) and local health authorities announced a seventh US coronavirus case had been confirmed in Northern California.

The latest US patient was identified only as a man in Santa Clara County, south of San Francisco, who became ill after travelling to China, and has "self-isolated" at home, Sara Cody, director of public health for the county, told reporters.

She said the CDC was seeking to determine whether the man was infectious while flying home.

In addition to the US entry ban on foreign travellers to China and quarantine orders for Americans returning from Hubei, Azar also said all commercial flights from China would be restricted to seven US airports.

"The actions we have taken and continue to take complement the work of China and the World Health Organisation (WHO) to contain the outbreak within China," Azar said.

CDC director Robert Redfield told reporters the US government acted after the WHO declared a global health emergency on Thursday over the spread of the respiratory disease.

"I want to emphasise that this is a serious health situation in China, but I want to emphasize that the risk to the American public is currently low," Redfield said. "Our goal is to do all we can do to keep it that way."

Nearly 200 Americans evacuated from China earlier this week and voluntarily confined to a military airbase in California for 72 hours of coronavirus screenings were placed under a mandatory 14-day quarantine on Friday.

The CDC's quarantine in California, the first imposed by the agency in 50 years, came a day after the state department issued its strongest warning against travel to China due to the coronavirus outbreak, which has claimed more than 200 lives. 

None of the US cases have been fatal, and all but one of the patients contracted it while they were travelling in China. - Reuters