Two more people in China have died from a new strain of avian influenza, bringing to 16 the number of deaths from the H7N9 virus, and the government has warned that the number of infections could rise.
The latest victims were from the commercial capital of Shanghai, where the majority of the 77 cases have been found, the official Xinhua news agency reported late on Tuesday.
The exact source of infection remains unknown and no human-to-human spread of the virus has been confirmed.
Samples have tested positive in some birds in poultry markets that remain the focus of investigations by China and the U.N.'s Food and Agriculture Organization (FAO).
"Because the source of the infection has not been effectively controlled, it is possible that the number of cases of infections could continue to rise," Xinhua said, quoting a risk assessment of the virus by the National Health and Family Planning Commission.
Zeng Guang, the chief scientist in charge of epidemiology at the China Disease Prevention and Control Centre (CDPCC), said about 40 percent of the victims have no clear history of poultry exposure, the Beijing News newspaper reported on Wednesday.
"How did these people get infected? It's a mystery," Zeng was quoted as saying.
According to a Reuters analysis of the infections, based on state media reports, only 10 out of the 77 victims as of Tuesday have had contact with poultry.
The CDPCC declined to comment when asked by Reuters.
The panic over bird flu has caused many Chinese to shun eating chicken for fear of catching the virus.
China's poultry sector has recorded losses of more than 10 billion yuan ($1.6 billion) since reports emerged of the new strain two weeks ago, an official at the country's National Poultry Industry Association said on Tuesday.
Authorities have slaughtered thousands of birds and closed live poultry markets in Shanghai and Beijing in an attempt to reduce the rate of human infection.
An international team of flu experts will go to China this week to help with investigations into the virus, the World Health Organization (WHO) said on Tuesday.
China said on Sunday the virus had spread outside the Yangtze River delta region in eastern China, with cases reported in the capital Beijing and the central province of Henan.