The WHO is holding a conference in Geneva on combatting the virus as Beijing struggles to contain a disease that has now infected more than 42,000 and reached some 25 countries.
Another 108 deaths were reported on Tuesday -- the first triple-digit daily rise since the virus emerged in late December.
"With 99 percent of cases in China, this remains very much an emergency for that country, but one that holds a very grave threat for the rest of the world," said WHO chief Tedros Adhanom Ghebreyesus at the conference, where scientists were discussing how the virus is transmitted and possible vaccines.
Chinese authorities have locked down millions of people in a number of cities, while several governments have banned arrivals from China and major airlines have suspended flights in a bid to keep the disease away from their shores.
The death toll has now reached 1,016, although the mortality rate remains relatively low at 2.4 percent.
But the case of a British man who passed on the virus to at least 11 other people -- without having been in China -- has raised fears of a new phase of contagion abroad.
- 'Super-spreader' -
Most cases overseas have involved people who had been in Wuhan, the quarantined central Chinese city where the virus emerged late last year, or people infected by others who had been at the epicentre.
But the Briton -- dubbed a "super-spreader" by some British media -- caught the virus while attending a conference in Singapore and then passed it on to several compatriots while on holiday in the French Alps, before finally being diagnosed back in Britain.
Of those infected, five were hospitalised in France, five in Britain and one on the Spanish island of Mallorca.
"The detection of this small number of cases could be the spark that becomes a bigger fire," Tedros said on Monday, urging countries to seize on the "window of opportunity" to prevent a bigger outbreak.
Michael Ryan, head of the WHO's Health Emergencies Programme, said it was "way too early" to call the Singapore conference a "super-spreading event".
- 'Imminent threat' -
As the number of cases in Britain doubled to eight, the government called the novel coronavirus a "serious and imminent threat", and said anyone with the disease could be forcibly quarantined if deemed a threat to public health.
The biggest cluster of cases outside China is aboard the Diamond Princess cruise ship moored off Japan, where 135 people have been positively diagnosed.
The ship has been in quarantine since arriving off the Japanese coast early last week after the virus was detected in a former passenger who disembarked last month in Hong Kong.
More than 100 people were evacuated from a 35-storey Hong Kong housing block Tuesday after two residents in different apartments tested positive for the virus.
Residents were forced to leave as health officials in masks and white overalls scrambled to work out whether the virus had spread through the complex of some 3,000 people.
- Torrent of criticism -
Chinese authorities, meanwhile, dismissed two senior health officials from Hubei, the central province where some 56 million people, including in its capital Wuhan, have been under lockdown since late last month.
They also tightened restrictions in the city, forbidding people with fever from visiting hospitals outside of their home districts and sealing off residential compounds.
Local authorities in Wuhan and Hubei have faced a torrent of criticism for hiding the extent of the outbreak in early January. Most deaths and cases are in Hubei.
The death of a whistleblowing doctor from Wuhan has sparked calls for political reform in China.
President Xi Jinping, who has described the battle against the virus a "people's war", has largely kept out of the public eye since the outbreak spiralled across the country from Hubei.
But he emerged on Monday, pictured wearing a mask and having his temperature taken at a hospital in Beijing where he spoke with medical staff and patients.
He called the situation in Hubei "still very grave" and urged "more decisive measures" to contain the spread of the virus.
Tellers in full protective masks process shopping at a supermarket in Shenyang in China's northeastern Liaoning province
Cumulative and daily tolls of coronavirus cases, with numbers of cases per country/territories outside mainland China, as of Feb 11, 0500 GMT.
A couple wearing face masks walk through a deserted shopping mall in Beijing
The biggest cluster of infections outside China is aboard the Diamond Princess cruise ship moored off Japan.