(Adds WHO comment, context, UK reports of cases, market reaction, color)
* China confirms 18 deaths, 634 virus cases by Thursday
* Wuhan shuts transport links, outgoing flights
* WHO says Wuhan lockdown of 11 million 'unprecedented'
GENEVA/BEIJING, Jan 23 (Reuters) - The World Health Organisation (WHO) said on Thursday it was "a bit too early" to declare a new coronavirus a global health emergency as China put millions of people on lockdown amid an outbreak that has killed 18 people in the country and infected around 650 globally.
Health officials fear the transmission rate could accelerate as hundreds of millions of Chinese travel at home and abroad during week-long holidays for the Lunar New Year, which begins on Saturday.
"It is a bit too early to consider that this is a public health emergency of international concern," WHO Emergency Committee panel chair Didier Houssin said after the body met in Geneva.
WHO chief Tedros Adhanom Ghebreyesus said the committee of 16 independent experts had been divided in its conclusion.
"Make no mistake, though, this is an emergency in China. But it has not yet become a global health emergency. It may yet become one," said Tedros. Declaring the disease a global health emergency would have required countries to step up the international response.
Scrambling to contain the outbreak, the local government in Wuhan, a city of 11 million people at the center of the outbreak, suspended most transport on Thursday and people were told not to leave. Hours later, neighbouring Huanggang, a city of about 7 million people, announced similar measures.
"The lockdown of 11 million people is unprecedented in public health history," said Gauden Galea, the WHO's representative in Beijing.
The organization said, however, that it was not yet recommending any broader restrictions on travel or trade.
"China has taken measures it believes is appropriate to contain the spread of coronavirus in Wuhan and other cities. We hope they will be both effective and short in their duration," said Tedros.
Peter Piot, a professor of global health and director of the London School of Hygiene and Tropical Medicine, said the outbreak was at a critical phase.
"Regardless of the decision not to declare this a Public Health Emergency of International Concern, intensified international collaboration and more resources will be crucial to stopping this outbreak in its tracks," he said. "There cannot be any complacency as to the need for global action."
The previously unknown virus strain is believed to have emerged late last year from illegally traded wildlife at an animal market in Wuhan, the capital of Hubei province in central China.
Chinese state television said 634 cases had been confirmed. By the end of Wednesday, China's National Health Commission confirmed 17 dead in Hubei. The first confirmed death outside Hubei - an 80-year-old man in Hebei, just south of Beijing - was also announced by health authorities.
Non-fatal cases have also been detected in Thailand, Vietnam, Singapore, Japan, South Korea, Taiwan and the United States.
Five people were being tested in Scotland for coronavirus as a precaution, the BBC reported on Thursday. All had respiratory symptoms and had recently been in Wuhan, it said.
The newly identified coronavirus has created alarm because there are a number of unknowns surrounding it. It is too early to know just how dangerous it is and how easily it spreads between people.
There is no vaccine for the virus, which can spread through respiratory transmission. Symptoms include fever, difficulty breathing and coughing, similar to many other respiratory illnesses.
Michael Ryan, head of the WHO Health Emergencies Programme, said data from China suggested almost three-quarters of the cases were in people aged over 40, with some 40% having underlying health conditions.
Three research teams are to start work on developing a vaccine, a global coalition set up to fight diseases said.
Preliminary research suggested the virus was passed on to humans from snakes, but Chinese government medical adviser Zhong Nanshan has also identified badgers and rats as possible sources.
The biggest tumble in Chinese stocks in more than eight months led global equity markets lower on Thursday as concern mounted about the outbreak.
In Wuhan, the Hankou railway station was nearly deserted on Thursday, state broadcasts showed. State media reported highway toll booths around Wuhan were closing, effectively cutting off road exits, and all ride-hailing services would be cancelled from Friday. Guards were patrolling highways, one resident said.
As the city slipped into isolation, residents thronged hospitals for medical checks and rushed to buy supplies, clearing out supermarket shelves and queuing for petrol.
Other cities were also taking steps to contain the virus.
Nearby Ezhou shut train stations. Beijing cancelled large gatherings, including two Lunar New Year temple fairs, and closed the Forbidden City, the capital's most famous tourist attraction, to visitors until further notice.
The U.S. State Department warned travellers to exercise increased caution in China as airports worldwide were screening passengers arriving from the country.
Chinese-ruled Hong Kong, which has two confirmed cases, is turning two holiday camps into quarantine stations as a precaution.
Taiwan has banned anyone from Wuhan from going to the island.
Chinese people had their own ways of protecting themselves.
"I go straight to where I need to go, and then I go home," said 79-year-old Li Meihua, from behind a mask, on the streets of Shanghai. "I'm also maintaining a cleaner diet. I've turned vegetarian."
(Reporting by Yawen Chen, Se Young Lee, Sophie Yu and Gabriel Crossley in Beijing, Sam Shen and Engen Tham in Shanghai, Ben Blanchard in Taiwan, Alison Lui and Donny Kwok in Hong Kong, John Geddie and Aradhana Aravindan in Singapore, Stephanie Nebehay in Geneva, and Kate Kelland and Elizabeth Howcroft in London; Writing by Alison Williams, Nick Macfie and Rosalba O'Brien; Editing by Janet Lawrence and Cynthia Osterman)