GENEVA, SWITZERLAND- A deadly virus outbreak in China and beyond does not yet constitute a global emergency, the World Health Organization said Thursday, urging China to keep its lockdown of some 20 million people "short".

"I am not declaring a public health emergency of international concern today," WHO chief Tedros Adhanom Ghebreyesus told reporters after a two-day emergency meeting in Geneva on the virus.

He stressed though that the decision to hold off on issuing the declaration used for the gravest epidemics "should not be taken as a sign that WHO does not think the situation is serious."

"This is an emergency in China, but it has not yet become a global health emergency," he told reporters.

The new respiratory virus has claimed 18 lives since emerging from a seafood and animal market in Wuhan, infected hundreds of other people nationwide and has been detected as far away as the United States.

The new virus has caused alarm because of its similarity to SARS (Severe Acute Respiratory Syndrome), which killed nearly 650 people across mainland China and Hong Kong in 2002-2003.

The WHO has confirmed that the new virus can be passed between people, at least those in close contact. Chinese health officials warned it could mutate and spread further.

But Tedros stressed there was so far "no evidence of human-to-human transmission outside China."

In response to the outbreak, China has effectively quarantined nearly 20 million people, and announced measures to curb the disease nationwide as hundreds of millions of people began travelling across the country this week for the Lunar New Year holiday.

Tedros hailed China for taking "measures it believes appropriate to contain the spread of coronavirus in Wuhan and other cities."

But he said "we hope that they will be both effective and short in their duration."