In its latest update, China's National Health Commission said it had recorded 121 new deaths and 5,090 new coronavirus cases on the mainland on February 13, taking the accumulated total infected to 63,851 people.
There are at least 570 confirmed cases of coronavirus in more than 25 countries and territories outside mainland China.
It was not immediately clear how the new methodology affected the results, nor why the death toll rose so sharply.
More than two-dozen countries have now reported hundreds of cases among them.
"This outbreak could still go in any direction", he told reporters. U.S. President Donald Trump praised China over its response and said Washington was working closely with Beijing.
"I suspect but can't be certain that the underlying trend is still downwards", Professor Hunter said. The new deaths also include eight who were clinically diagnosed.
The revision added almost 15,000 patients to Hubei's count in a single day, with officials explaining that past cases were included.
Some 55,748 people are now undergoing treatment, while 1,380 people have died of the flu-like virus that emerged in Hubei province's capital, Wuhan, in December.
Covid-19 confirmed cases
The WHO said the numbers included cases going back weeks.
The sharp one-day increase "does not represent a significant change in the trajectory of the outbreak", said Michael Ryan, head of WHO's health emergencies programme.
But the 2,015 new confirmed cases reported in mainland China on Wednesday was dwarfed by the 14,840 new cases reported in Hubei alone on Thursday when provincial officials said they had adopted a new methodology for counting infections. Of those cases, more than 3,000 were "clinically diagnosed".
Japanese policymakers vowed to step up testing and containment efforts after the death and confirmation of new cases, including a doctor and a taxi driver.
Some cities in Hubei tightened restrictions this week, sealing off neighbourhoods in what they liken to "war-time" measures.
Australian health officials tested a passenger onboard another cruise ship that docked in Sydney harbor for a "respiratory illness" on Friday, causing passengers to fret about the potential of another shipboard outbreak of the coronavirus.
Several countries have banned arrivals from China, while major airlines have halted flights to and from the country.
"We thought there was better transparency coming out of China, but it doesn't appear to be", Kudlow said.