BEIJING (AP) — The Chinese city of Wuhan shut down outbound flights and trains as the world's most populous country battled the spread of a new virus that has sickened hundreds of people and killed 17, Chinese state media said early Thursday.

The state-owned People's Daily newspaper said no one would be allowed to leave the city of several million people. The official Xinhua News Agency said no one would be permitted to leave without a specific reason.

Most of the cases are in Wuhan and surrounding Hubei province, but dozens of infections have popped up this week around the country as millions travel for the Lunar New Year holiday, one of the world's largest annual migrations of people.

In Geneva, the World Health Organization put off deciding whether to declare the outbreak a global health emergency and asked its expert committee on the issue to continue meeting for a second day Thursday. The organization defines a global emergency as an "extraordinary event" that constitutes a risk to other countries and requires a coordinated international response.

The first case in the U.S. was reported Wednesday in Washington state. Airports in big cities including Chicago and Atlanta have started screening people arriving from China for the virus.

Other countries have also stepped up screening measures for travelers from China, especially those arriving from Wuhan.

The number of new cases has risen sharply in China, the center of the outbreak. The 17 deaths announced Wednesday night were all in Hubei province, where the outbreak emerged late last month. Wuhan authorities said the province has confirmed 444 cases, which would bring the national total to more than 500.

"There has already been human-to-human transmission and infection of medical workers," Li Bin, deputy director of the National Health Commission, said Wednesday. "Evidence has shown that the disease has been transmitted through the respiratory tract, and there is the possibility of viral mutation."

The illness comes from a newly identified type of coronavirus, a family of viruses that can cause the common cold as well as more serious illnesses such as the SARS outbreak that spread from China to more than a dozen countries in 2002-2003 and killed about 800 people. Some experts have drawn parallels between the new coronavirus and Middle Eastern respiratory syndrome, another coronavirus that does not spread very easily among humans and is thought to be carried by camels.

But WHO's Asia office tweeted this week that "there may now be sustained human-to-human transmission," which raises the possibility that the epidemic is spreading more easily and may no longer require an animal source to spark infections, as officials initially reported.

Authorities in Thailand on Wednesday confirmed four cases — a Thai national and three Chinese visitors. Japan, South Korea, the United States and Taiwan have all reported one case each. All of the illnesses were of people from Wuhan or who recently traveled there.

Travel agencies that organize trips to North Korea said the country has banned foreign tourists because of the outbreak.

This report includes material from the Washington Post.

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