"The main reason for this declaration is not what is happening in China but what is happening in other countries," said WHO chief Tedros Adhanom Ghebreyesus.

The concern is that it could spread to countries with weaker health systems.

The death toll now stands at 170 people in China.

The WHO said there had been 98 cases in 18 countries outside of the country, but no deaths.

Most cases have emerged in people who have travelled from the Chinese city of Wuhan, where the outbreak began.

However, there have been eight cases of human-to-human infection - in Germany, Japan, Vietnam and the United States.

Dr Tedros, speaking at the press conference in Geneva, described the virus as an "unprecedented outbreak" that has been met with an "unprecedented response".

He praised the "extraordinary measures" Chinese authorities had taken to prevent it from spreading.
"Let me be clear, this declaration is not a vote of no confidence in China," he added.

BBC Health correspondent James Gallagher said the WHO will now be able to support lower and middle income countries, helping them strengthen their disease surveillance and prepare them for possible cases.

How unusual is this declaration?

The WHO declares a Public Health Emergency of International Concern when there is "an extraordinary event which is determined … to constitute a public health risk to other states through the international spread of disease".

It has previously declared five global public health emergencies: