Two people have died from the latest strain of bird flu in China, bringing to 16 the number of deaths from the H7N9 virus.

Both victims came from Shanghai where the majority of cases have been found.

The source of the infection remains unknown.

So far there've been no reports of the virus spreading from person to person.

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.