Some 242 deaths from the new coronavirus were recorded in the Chinese province of Hubei on Wednesday, the deadliest day of the outbreak.
There was also a huge increase in the number of cases, with 14,840 people diagnosed with Covid-19.
Hubei has started using a broader definition to diagnose people, which accounts for most of the rise in cases.
China sacked two top officials in Hubei province hours after the new figures were revealed.
Until Wednesday's increases, the number of people with the virus in Hubei, where the outbreak emerged, was stabilising.
But the new cases and deaths in the province have pushed the national death toll above 1,350 with almost 60,000 infections in total.
Meanwhile Japan has announced its first coronavirus death - a woman in her 80s who lived in Kanagawa, south-west of Tokyo. It is the third death outside mainland China, following one each in the Philippines and Hong Kong.
The woman's diagnosis was confirmed after her death and she had no obvious link to China's Hubei province, Japanese media reported.
The World Health Organization (WHO) says it is seeking "further clarity" from China about the changes to how cases of the virus are being confirmed.
China has been accused of suppressing the full extent of the outbreak in the past, says the BBC's Nick Beake in Hong Kong.
David Heymann, Professor of Infectious Disease Epidemiology from the London School of Hygiene and Tropical Medicine, said: "What has happened in China is that they have changed the definition of what the disease really is - now they are taking people who have lesser symptoms.
"The deaths are quite worrisome, there is an increased number of deaths reported, but if you look overall at the total number of deaths and the total number of cases, the fatality ratio is about the same as it has been - but it is still high, as high as the death rate in influenza."
Only Hubei province - which accounts for more than 80% of overall Chinese infections - is using the new definition to diagnose new cases.
Can we trust the numbers?
Just about everyone who's been following China's official coronavirus numbers has been able to see that they have been incomplete. Government officials know this too. There's no way they've accounted for everybody infected. How could they?
But at least we had what appeared to be a trend. We could observe the pattern to try and estimate the trajectory of outbreak. Now that's gone too.
You can understand why it has been decided that people who have virus symptoms, plus a CT scan showing chest infection, are now being counted in the "definitely infected" column. However, this has thrown the trend mapping into chaos.
Over the past 24 hours in Hubei alone, nearly 15,000 people were moved into the infected column. This would have sent shockwaves around the world, but actually, if you consider Wednesday's cases by the old definition, the rate could well mean another day of decline: a completely different picture.
So now, we're scratching our heads: do we start looking at the pattern all over again from Thursday onwards? This has also left many wondering what the real death rate must have been over recent weeks and the extent to which we should be treating the overall figures seriously anyway.
Meanwhile, the Communist Party secretary in Hubei, Jiang Chaoliang, has been replaced by the Shanghai party chief, Ying Yong, according to state media. The party chief of the capital city, Wuhan, has also been relieved of his duties.
It is the first major change of Hubei party officials since the outbreak began.
Earlier this week, a number of health officials were "removed" from their jobs.
What is the new diagnosis method?
The province now includes "clinically diagnosed cases" in the number of confirmed cases.
This means it includes those showing symptoms, and having a CT scan showing an infected lung, rather than relying only on the standard nucleic acid tests.
Of the 242 new deaths in Wuhan, 135 are such "clinically diagnosed" cases.
That means, even without the new definition, the number of deaths in Hubei on Wednesday was 107 - a new high for the province.
The province's 14,840 new infections include 13,332 clinically diagnosed cases.
Overall, the province now has 48,206 confirmed infections.
What is the latest with the cruise ships?
A cruise ship carrying more than 2,000 people has docked in Cambodia after it was turned away by five ports over fears that some passengers might be infected with the virus.
The MS Westerdam arrived on Thursday morning after Japan, Taiwan, Guam, the Philippines and Thailand had all refused to accept the ship despite having no sick patients on board.
Meanwhile, another 44 cases have been confirmed on the Diamond Princess, which is in quarantine in Yokohama, Japan.
The increase means 218 people of the 3,700 people on board the ship have caught the virus. Not everyone has been tested yet.
People with the virus are taken to hospitals on land to be treated, while those on board are largely confined to their cabins.
However on Thursday Japan said it would allow those aged 80 or over who have tested negative for the coronavirus to disembark.
Health Minister Katsunobu Kato said they could be allowed off the ship as early as Friday but would have to stay in accommodation provided by the government, the Japan Times reported.
In other developments:
- In the UK, officials are attempting to trace the contacts of the latest person to be diagnosed with coronavirus. The woman, who flew into London Heathrow from China a few days ago, is the ninth case to be confirmed
- Australia has extended its ban on people coming from mainland China for another week, to 22 February from 15 February
- Hong Kong's most high-profile sports event, the Rugby Sevens, as well as the Singapore Sevens, are expected to be postponed
- China said it would stagger the return of children to school - Several provinces have closed schools until the end of February
- In Vietnam, which borders China, thousands of people in villages near the capital, Hanoi, have been put under quarantine after several cases were discovered. Vietnam has now confirmed at least 16 cases
- The Red Cross has called for sanctions relief for North Korea, which would allow the aid agency to transfer funds to buy equipment. Testing kits and protective clothing are urgently needed to prepare for a possible outbreak, it says
- British rap star Stormzy has postponed the Asian leg of his tour - he had been due to play in locations including Shanghai, Hong Kong, Singapore and Malaysia in March
Read more about the coronavirus and its impact
SHOULD WE WORRY? Our health correspondent explains
YOUR QUESTIONS: Can you get it more than once?
WHAT YOU CAN DO: Do masks really help?
UNDERSTANDING THE SPREAD: A visual guide to the outbreak
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