The US government has advised against all travel to China due to the threat posed by the coronavirus outbreak, raising its alert to the highest level.
The state department's "do not travel" warning, issued for extremely dangerous cases, was announced after the World Health Organization (WHO) declared the outbreak a global emergency.
Those in China were urged to "consider departing using commercial means".
At least 213 people have died in China, with almost 10,000 cases of the virus.
There have been 98 cases in 18 other countries, according to the WHO, but no deaths. Most international cases are in people who had been to the Chinese city of Wuhan, where the outbreak began.
In its updated advisory, the state department said it had "requested that all non-essential US government personnel defer travel to China". Last week, it ordered all non-emergency personnel and their family members to leave Wuhan.
This alert is the highest of the four-level warning system. The previous advisory had told people only to "reconsider" travel to China.
Earlier on Thursday, health officials in Chicago reported the first case of human-to-human transmission of the virus in the United States. The discovery marked the second report of the virus in Illinois and the sixth confirmed case in the US.
The new patient, a 60-year-old male, apparently contracted the virus from his spouse, a Chicago woman who carried the infection back from Wuhan, the US Centers for Disease Control and Prevention (CDC).
The man had "some underlying medical conditions" but was in good condition, officials said. His wife, who had been caring for her father in Wuhan earlier this month, was also stable but remained in isolation at a local hospital.
Officials were also monitoring 165 patients across the US for possible infections but CDC director Dr Robert Redfield cautioned the public to remain calm. "Our assessment remains that the immediate risk to the American public is low," he said.
The American Airlines pilots' union said it was suing the airline in an attempt to stop flights between the US and China. In a statement, the union said it had instructed members to turn down requests to fly to China.
Speaking in Geneva, WHO chief Tedros Adhanom Ghebreyesus said the global emergency declaration was made because of concerns that the virus could spread to countries with weaker health systems.
He praised the "extraordinary measures" Chinese authorities had taken, and said there was no reason to limit trade or travel to China. "Let me be clear, this declaration is not a vote of no confidence in China," he said.
A number of countries have implemented evacuation and quarantine plans for nationals wanting to return from China, and Russia closed its 4,300km (2,670-mile) far-eastern border with China.
Flights to China have been cancelled and companies like Google, Ikea, Starbucks and Tesla have closed their shops or stopped operations.
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