World Health Organization experts have arrived in China to study the spread of H7N9 avian influenza and possible human transmission of the virus, a WHO official said on Friday.

Chinese and international experts suspected human transmission of H7N9 “in very rare cases,” Michael O’Leary, WHO China representative, told reporters.

The WHO team would spend a week in China, travelling to areas where infections were reported and making recommendations to the government, Mr O’Leary said.

The experts would focus on three clusters of infections in eastern China, including a father and two sons, he said.

The two sons of an 87-year-old Shanghai man who died of H7N9 were both treated for severe pneumonia, from which one of them died.

Officials said tests on the sons in early April proved negative for H7N9, but experts are re-examining the three cases for possible human transmission of the virus.

“It is not rare that with animal-to-human (transmission) you also have rare human-to-human transmissions,” Mr O’Leary said.

President Xi Jinping on Friday ordered local health departments to “release accurate information about the disease to maintain order”.

China reported 88 confirmed H7N9 infections, including 17 deaths, by late Friday.

Most of the cases were in Shanghai and the neighbouring provinces of Anhui, Jiangsu and Zhejiang.

One new infection was confirmed on Friday in a 54-year-old man in Nanjing, the Jiangsu provincial capital, local authorities said.

Mr O’Leary said evidence so far suggested that poultry were a vehicle of transmission to people, but experts had not confirmed a “strong link.” Chinese scientists found no evidence of direct transmission from wild birds to people, state media said on Friday.

But the government still suspended sales of wild birds at animal markets from Thursday and ordered zoos to prevent close contact between humans and animals, the official Xinhua news agency said.

An earlier report by the Caixin financial newspaper said microbiologists had identified possible viral mutation from wild birds that migrated from South Korea and mingled with ducks and chickens in eastern China.