Isolating the outbreak epicentre of Wuhan, and reducing flights to and from the worst-affected region, have limited potential further spread, the news conference heard.
Addressing scientists at the first worldwide conference on combating the virus in Geneva on Tuesday, WHO chief Tedros Adhanom Ghebreyesus warned that the virus was a "very grave threat".
And Dr Mike Ryan, the head of the WHO's emergency programme, has cautioned that it is too early "to predict the beginning, the middle or end of the epidemic".
The total deaths in mainland China since the outbreak began in December stood at 1,367, with the total number of confirmed cases mounting to 52,526.
China also appointed new high-level officials in Hubei and Wuhan.
"One further patient in England has tested positive for the novel coronavirus (COVID-19), bringing the total number of cases in the United Kingdom to nine", Chris Whitty, chief medical officer for England said in a statement.
A second person evacuated from Wuhan to a USA military base near San Diego has been diagnosed with the new coronavirus, raising the tally of confirmed cases in the United States to 14, the Centers for Disease Control and Prevention (CDC) said on Wednesday. This figure now includes more than 13,000 cases of "clinical diagnosis", which appears to include those based on a doctors' analysis combined with lung imaging, as opposed to waiting for laboratory test results.
About 56 million people in Hubei and its capital, Wuhan, are being banned from leaving as part of the quarantine efforts.
The world must "wake up and consider this enemy virus as public enemy number one", WHO chief Tedros Adhanom Ghebreyesus said on Tuesday, adding the first vaccine was 18 months away.
But it has faced continued scepticism among the global public, and U.S. officials have also called for more openness from China's Communist Party rulers, leading to fears that there may be similarities with the way it dealt with SARS.
Authorities in Hubei have been accused of concealing the gravity of the outbreak in late December and early January.
Hubei's two top health officials had already been sacked this week.
Analysts said Hubei's new methodology to count infections might be for medical reasons and could be because Xi wants officials to be more transparent, but the immediate impact was to sow more distrust.
"I don't think the numbers are necessarily manipulated for political purposes but the numbers themselves may not be so trustworthy", Yun told AFP.
Japan's Health Ministry announced Thursday that 44 more people on a cruise ship quarantined in the port of Yokohama, near Tokyo, have tested positive for COVID-19.
A security guard stands outside a plastic tent set up to disinfect people coming in at the entrance of a residential compound, as the country is hit by an outbreak of the new coronavirus, in Beijing, China, on February 13, 2020.
United Airlines said it would extend cancellations of all USA flights to China through late April, meaning all US passenger carriers flying to China had now cancelled flights into late April.
The outbreak has wreaked havoc on global events, too.
The Mobile World Congress, an annual telecoms industry gathering that draws more than 100,000 visitors to Barcelona, was cancelled on Wednesday after a mass exodus by exhibitors due to coronavirus-related concerns.
The Formula 1 Chinese Grand Prix, set for Shanghai on April 19, was postponed, as was Singapore's rugby sevens tournament scheduled for April 11-12, a source said.