Chinese authorities reported four new cases on Tuesday of a strain of bird flu previously unknown in humans that has already killed two people, taking the total of known cases to seven.
The World Health Organisation (WHO) said on Monday there was no evidence that the H7N9 strain could be transmitted between people, but that it was investigating the outbreak.
The four new patients in China's eastern Jiangsu province were all in critical condition and receiving emergency treatment, the Xinhua News Agency said, citing the Jiangsu provincial health bureau. A woman in Anghui province who caught the virus in early March is also in critical condition.
The Chinese news website Chinanews.com said the four patients in Jiangsu - a province next to Shanghai - were aged between 32 and 83 and only one, a 45-year-old woman, had worked in the market slaughtering poultry.
All four reported varying symptoms of dizziness, fever, cough and breathlessness.
China's National Health and Family Planning Commission on Sunday confirmed a Xinhua report that three people had been infected with the new strain.
The two deaths were men in Shanghai aged 87 and 27 who fell sick in late February.
It is not known how the seven victims were infected, though the government believes the virus is not highly contagious.
Xinhua said 43 people who had been identified as having been in close contact with the four patients in Jiangsu were under medical observation but had not developed flu symptoms.
The WHO said on Monday that the first three cases had shown no evidence of human-to-human transmission, but that there were questions to answer about the source of the infection and the mode of transmission.
China has a bad record when it comes to dealing with bad news, which is often covered up by officials fearing it may attract unwanted attention from superiors and damage promotion prospects, despite government efforts to enhance transparency.
In 2003, Beijing initially tried to cover up the epidemic of Severe Acute Respiratory Syndrome, which emerged in China and killed about a tenth of the 8,000 people it infected worldwide.
Some Chinese have complained that authorities took too long before announcing the deaths on Sunday, though the WHO says the government acted properly.
Wu Fan, chief doctor and director general of the Shanghai Municipal Center for Disease Control, told reporters on Tuesday that the government had acted as quickly as it possibly could.
"In this situation, to take 20 days to identify and confirm a new virus is already considered short," she said.
"We cannot say for certain or confirm if it was a case of a human catching an avian virus or an avian virus changing and becoming a new human flu virus."
Ian Jones, a professor of virology at Britain's University of Reading, said at this stage there was no cause for alarm.
"At the moment I don't think it's anything more than an unusual set of isolated cases," he told Reuters.
He said three types of avian influenza - H5, H7 and H9 - were considered by experts to be a potential threat to humans.
Since there is no evidence to date of human-to-human transmission, or of clusters of cases around those few confirmed so far, he said authorities should be watchful but need not enact emergency measures.
"Of course we need to take account of these cases and follow up the contacts and so on, but I think that's where it rests at the moment," he said. "It's far too soon to assume this is the start of something."