The coronavirus outbreak in Wuhan may have been wider in December 2019 than first thought, the World Health Organization (WHO) has said.
Peter Ben Embarek, the lead investigator of the recent WHO mission to the Chinese city in Hubei province, said there were signs of a wider outbreak.
He told CNN there may have been many more cases than initially reported at the end of 2019 when COVID-19 began to take hold.
The WHO mission also revealed there were already 13 different strains of coronavirus in Wuhan in December 2019.
He said on Monday: "The virus was circulating widely in Wuhan in December, which I think is a new finding."
Watch: WHO investigators find evidence that coronavirus was spreading earlier
Ben Embarek said the WHO team had been presented by Chinese scientists with 174 coronavirus cases dating from December 2019, but that there could have been more than 1,000 that went undetected.
"We haven't done any modelling of that ourselves but with what we know the big ballpark figure out of the infected population, you have about 15% that end up severe cases, and the vast majority are mild cases."
The WHO said more than a dozen strains had been identified from that period, although not all were from Huanan seafood market which has been linked to the spread of COVID-19.
“We have 13 strains covering individuals in December,” said Ben Embarek. “Some of them are from the market. Some of them are not linked to the market.
"This is something we have found as part of our mission.”
Scientists believe the large number of variants meant the disease was in wider circulation in China for longer than previously thought.
Ben Embarek said his team are still trying to access hundreds of thousands of blood samples dating back two years from Wuhan’s blood donor bank.
"There is about 200,000 samples available there that are now secured and could be used for a new set of studies,” he said.
”That would be fantastic if we could work with that."
Watch: WHO experts head to Wuhan to find COVID origins
The WHO mission also interviewed the first COVID-19 patient that China was aware of, a Wuhan man in his 40s who had no recent travel history.
"He has no link to the markets," said Ben Embarek. "We also spoke to him. He has a very - in a way - dull and normal life, no hiking in the mountains type of things. He was an office worker in a private company."
Ben Embarek spoke of the pressure on his team of scientists, who completed their 28-day mission to Wuhan last week.
“We've had the entire planet on our shoulders 24 hours a day for a month, which doesn't make the work among scientists easier,” he said.
His team has said wildlife on sale in the Huanan seafood market could be traced to regions that contain bat habitats known to harbour viruses closely related to COVID-19.
One such region is the Chinese province of Yunnan, but the team is also considering that the first human transmission took place across the border in Laos or Vietnam.
The WHO believes the Huanan market was linked to the first case of clusters, but that the initial crossover from animals to humans did not occur there.
China’s National Health Commission said there is insufficient evidence to determine how the virus entered Huanan, but that it was circulating elsewhere in Wuhan at the same time.
“The possible path from whatever original animal species all the way through to the Huanan market could have taken a very long and convoluted path involving also movements across borders,” said Ben Embarek last week.
Watch: Wuhan COVID investigator not sure virus originated there