China has extended travel restrictions to two more cities as part of an unprecedented quarantine to try and control a coronavirus that has killed at least 17 people.

The city of Wuhan, where the virus originated, shut down transport links inside the city on Thursday, leaving 11 million people cut off and stockpiling food and fuel.

Now the city of Huanggang has joined the quarantine, and the city of Ezhou is closing its train stations. This means a total of 19 million people are now cut off from the rest of the world.

The World Health Organization said cutting off a city as large as Wuhan is "unprecedented in public health history," and isn't sure if the strategy will work.

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China extended has extended its quarantine on the city of Wuhan over a deadly virus to two more cities on Thursday, cutting off a total of almost 19 million people.

Wuhan, the epicenter of the outbreak, shut down planes, trains, and buses from the city at 10 a.m. local time on Thursday morning, with passengers leaving before that time getting health screenings.

Residents and journalists also reported that roads out of the city had been closed. Wuhan is home to 11 million people. (New York City, by comparison, is home to 8 million people.)

Later on Thursday the city of Huanggang — a city of around 7 million people near Wuhan — announced it was adopting similar measures.

Huanggang officials have ordered for places like cinemas and cafes to close, and said citizens should not leave the city unless there are special circumstances, Reuters reported.

And the city of Ezhou, which is also nearby, announced that it is closing its train stations, Reuters reported. Ezhou is home to around 1 million people.

A largely empty train travels to Wuhan from Shanghai on January 23, 2020. HECTOR RETAMAL/AFP via Getty Images

This puts a total of 19 million people under some sort of travel restriction over the virus, which has killed 17 people and infected more than 570 people.

'Unprecedented in public health history'

The World Health Organization (WHO) has portrayed the idea of cutting off cities as unprecedented.

"To my knowledge, trying to contain a city of 11 million people is new to science," Gauden Galea, the WHO's China representative told the Associated Press.

"It has not been tried before as a public health measure, so we cannot at this stage say it will or will not work."

The three cities where quarantine efforts started on Thursday. Google Maps/Business Insider

Galea also told Reuters: "The lockdown of 11 million people is unprecedented in public health history, so it is certainly not a recommendation the WHO has made."

People left in Wuhan have described people stockpiling food and fuel, skyrocketing prices, and largely empty streets as the city comes to grips with its isolation and the disease.

Medical staff members carry a patient into the Jinyintan hospital, where patients infected by a mysterious SARS-like virus are being treated, in Wuhan in China's central Hubei province on January 18, 2020. STR/AFP via Getty Images

The 2019-nCoV virus was first reported in Wuhan at the end of last year, and has now spread to other parts of China, as well as the US, Taiwan, Japan, South Korea, and Thailand.

The virus can spread between humans, and has infected health workers working with patients, as well as people who came in close contact with infected people. Scientists on Wednesday said the disease may have originated from snakes.

This map shows where it had spread by Wednesday morning, and excludes the single case in the US:

Many airports around the world are ramping up their health screening around the world, and say they are ready to put people showing symptoms to quarantine zones.

Patients, and those identified as having been close to them, are also being kept in quarantine around the world.

The virus, believed to have come from animals, has pneumonia-like symptoms. Authorities warn that it is mutating, making it harder to study and treat quickly.