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.