The World Health Organization said on Wednesday that a number of people who have tested positive for a new strain of bird flu in China have had no history of contact with poultry, adding to the mystery about the virus that has killed 16 people to date.

WHO spokesman Gregory Hartl confirmed that "there are people who have no history of contact with poultry", after a top Chinese scientist was quoted as saying that about 40 percent of those with the H7N9 virus had had no contact with fowl.

"This is one of the puzzles still (to) be solved and therefore argues for a wide investigation net," Hartl said in emailed comments, though he added he did not know the exact percentage.

China has warned that the number of infections could rise from the current 77. The latest victims are from the commercial capital of Shanghai, where the majority of the cases have been found, the official Xinhua news agency said on Tuesday.

China reported three new outbreaks to the World Animal Health Organization (OIE) this week, bringing the total number of places to 11, the OIE said on its website.

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 poultry markets that remain the focus of investigation by China and the U.N.'s Food and Agriculture Organization.

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 had no clear history of poultry exposure, the Beijing News 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 of the 77 cases as of Tuesday have had contact with poultry.

The CDPCC declined to comment when asked by Reuters.

WHO said no H7N9 vaccine was currently being produced. "Such a decision will depend, for WHO's part, on public health considerations," it said. "We must see how H7N9 develops before any such decision is considered."

The U.S. Centers for Disease Control and Prevention (CDC) said in early April it had started work on a vaccine just in case it was needed.

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, 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.