The U.S. State Department increased its travel advisory for China on Thursday, as the death toll from a fast-spreading virus rose to 213 and the World Health Organization (WHO) declared a global health emergency.

In Hubei, the Chinese province at the centre of the epidemic, the death toll had risen to 204 and there were 9,692 cases of infection nationally as of Thursday, Chinese health authorities said.

About 100 cases have been reported in 19 other countries; Italy was the latest to announce its first confirmed cases, in two Chinese tourists. There have been no deaths outside China.

Italian Prime Minister Giuseppe Conte said the government had decided to close all air traffic between Italy and China, a more drastic measure than most countries have undertaken.

Zhang Jun, China's ambassador to the United Nations, said China is "still at a very critical stage in fighting the coronavirus," in response to the WHO's declaration. "International solidarity is extremely important and for that purpose all countries should behave in a ... responsible manner."

"While we understand the concerns of other countries, we should also listen to the advice of [the] WHO director-general," Zhang said. "He said he had full confidence in China's efforts."

Zhang noted that it was important to avoid "overreactions which may result in spillover negative effects."

Countries began isolating hundreds of citizens evacuated from the Chinese city of Wuhan on Thursday to stop the spread of the epidemic as worry about the impact on the world's second-biggest economy rattled markets.

Asian stocks and currencies fell as the death toll rose and more cases were reported with fears growing that the hit to China's economy will ripple around the world in coming months.India became the latest country to report a case — a Wuhan University student — while anger and fear brought protests in South Korea and threats of strikes in Hong Kong.

Three new cases emerged in Vietnam, the official health ministry newspaper said Thursday, bringing the number of active cases in the Southeast Asian country to four.

"Markets will remain highly volatile as long as they feel that they only have an incomplete picture of what is going on, and what is going to happen next," said Agathe Demarais, global forecasting director at the Economist Intelligence Unit.

A 3D rendering of the 2019 Novel Coronavirus (2019-nCoV), identified as the cause of an outbreak of respiratory illness first detected in Wuhan, China. (Alissa Eckert/MS;/Dan Higgins/MAM/CDC/Reuters)

In a sign of alarm over possible damage, Bank of Japan Deputy Governor Masayoshi Amamiya said China's huge presence in the world economy must be taken into account in gauging the impact the outbreak could have on global growth.

Alphabet Inc.'s Google and Sweden's IKEA joined other major firms in closing operations in China.

The International Monetary Fund (IMF) said Thursday that it is closely monitoring the outbreak, but it is too soon to quantify the potential economic impact there, which is halting tourism and commerce throughout the country.

IMF spokesperson Gerry Rice said the direct impact on consumer and business demand had been most severe in Hubei province, and the extent of the impact depends on how quickly the virus subsides.

Concern is also growing that thousands of Chinese factory workers on Lunar New Year holidays may struggle to get back to work next week, due to extensive travel restrictions imposed to stop the spread of the virus.

South Korea's Samsung Electronics Co. Ltd. said it had extended the holiday closure for some Chinese production facilities.

WATCH: Shanghai steps up measures at railway stations to control spread of coronavirus

Travellers arriving or leaving undergo checks for high temperatures by medical staff. 0:33

Cases of human-to-human transmission concern WHO

Almost all the deaths have been in the central province of Hubei, home to about 60 million people and now under virtual lockdown. The virus emerged last month in a live wild animal market in the provincial capital of Wuhan.

Infections have been reported in at least 18 other countries, with 83 confirmed cases, but no deaths have occurred outside China.

Excavators and workers are seen at the construction site where the new Huoshenshan Hospital is being built to treat patients of the new coronavirus on the outskirts of Wuhan. (China Daily/Reuters)

Cases of human-to-human transmission outside China are of particular concern to the WHO, but it is too early to determine how lethal the virus is, as there are likely to be many cases of milder infections going undetected.

It has an incubation time of between one and 14 days, and there are signs it may be able to spread before any symptoms show.

Chinese National Health Commission Minister Ma Xiaowei said this week the virus was infectious during incubation, unlike severe acute respiratory syndrome (SARS), another coronavirus that emerged from China and killed about 800 people in 2002 and 2003.

More than 6,000 tourists were under lockdown aboard the Costa Smeralda cruise ship after two Chinese passengers were isolated over fears they could be carrying the coronavirus. (Filippo Monteforte/AFP/Getty Images)

On Thursday, countries began isolating hundreds of citizens evacuated out of China.

Australia, South Korea, Singapore, New Zealand and Indonesia will quarantine evacuees for at least two weeks, while the United States and Japan plan shorter periods of voluntary isolation.

Australia defended a decision to put evacuees on Christmas Island, 2,600 kilometres off its coast.

An Indian official said bringing citizens home was not the best option because of the risk of infection — but pressure from citizens there, most of them students and their families, had forced the government to put an aircraft on standby.

In South Korea, protesters who want facilities earmarked as quarantine centres moved away from residential areas threw eggs and insults at a minister trying to mollify them.

An employee wears goggles as she works at Hong Kong West Kowloon High Speed Train Station Terminus, before its temporary closing, following the coronavirus outbreak in Hong Kong on Wednesday. (Tyrone Siu/Reuters)

President Moon Jae-in has urged people not to give in to fear as the country prepared to evacuate the first of about 700 citizens from Wuhan.

"The weapons that will protect us from the new coronavirus are not fear and aversion, but trust and co-operation," he said.

Up to four planned South Korean evacuation flights to Wuhan are expected to begin later Thursday.

Singapore said all the 92 citizens it is bringing home on Thursday, with accompanying foreign ministry staff, will be taken either to hospital or quarantined for 14 days.

U.S. launches coronavirus task force

U.S. President Donald Trump's administration announced the formation of a task force to lead the U.S. response as it prepared to evacuate more citizens from Wuhan. It will lead efforts to monitor, contain and mitigate the spread of the virus, while ensuring Americans have the most accurate and up-to-date health and travel information.

Nearly 200 Americans, mostly diplomats and families, airlifted from the city on Wednesday, will stay isolated at a U.S. military base in California for at least 72 hours, officials said.

The U.S. government will provide extra evacuation flights from Wuhan with capacity for private U.S. citizens next week, the State Department said in a statement Thursday.

Three Japanese people, from 206 evacuated on Wednesday, were found to be infected, and two of them had not shown symptoms, the health ministry said.

A second Japanese flight landed in Tokyo on Thursday with nine people showing symptoms of fever or coughing, broadcaster NHK said.

In China, a government economist said first-quarter growth could fall by one point to 5 per cent or lower.

A worker uses a thermometer to check the temperature of a customer as she enters a Starbucks shop in Beijing on Thursday. (Carlos Garcia Rawlins/Reuters)

Some airlines have suspended flights to China, including British Airways, Lufthansa, Air Canada and American Airlines.

Russia will temporarily stop issuing electronic visas to Chinese nationals from Thursday, the foreign ministry said. Russia will also limit its railway links with China from Friday, leaving only direct trains between Moscow and Beijing.

Streets in many Chinese cities are largely deserted and tourist attractions are shut. Starbucks coffee shops require temperature checks and masks.