Cities in eastern China where an H7N9 bird flu outbreak has killed six people moved Saturday to prevent the virus from spreading by banning live poultry trade and culling fowl.

Nanjing city shut markets selling live poultry to its more than eight million residents, while Hangzhou culled birds after discovering infected quail, the official Xinhua news agency said.

Two more people were confirmed to be infected with the virus in Shanghai, state media said late Saturday, bringing to 18 the total number of cases since authorities last week announced the virus had been detected in humans.

The human infections have been confined to eastern China, with Shanghai recording eight including four deaths, and the other two fatalities in the neighbouring province of Zhejiang.

Other cases are scattered across the provinces of Jiangsu and Anhui.

Shanghai had ordered a ban on live poultry trading and markets after culling more than 20,500 birds at an agricultural market in a western suburb on Friday.

At a local market in the city centre, two live poultry booths were dark and the cages within empty, as a uniformed worker sprayed disinfectant from a tank on his back.

"People are worried," said Yan Zhicheng, a retired factory manager who like many elderly people makes a daily trip to market.

"Shanghai people eat a lot of duck and chicken. Now we can't touch them."

Shanghai has also banned live poultry from other parts of China entering the city and the sale of wild birds, including those intended as pets, the local government said in a statement on its website.

But eggs remain on sale in the city, as well as fresh and frozen poultry meat, though officials encourage people to cook them well.

Chinese authorities maintain there is no evidence of human-to-human transmission, a conclusion echoed by the World Health Organization (WHO).

The China Food and Drug Administration announced Saturday that it had approved a flu drug called Peramivir, which it believes may be effective in treating the H7N9 virus, according to Xinhua.

State media said the government had sought to improve transparency on the disease after being accused of covering up the 2003 outbreak of Severe Acute Respiratory Syndrome (SARS), which killed about 800 people globally.

"The government's response... is completely different from 10 years ago, when information disclosure systems were not established," Wang Yukai of the Chinese Academy of Governance told Xinhua.

But some of China's outspoken Internet users remained sceptical of government assurances.

"Get out of here. If it is not infectious then what are you doing shutting live poultry markets?" said Huang Kekedou in a microblog posting.

The US Centers for Disease Control and Prevention on Friday warned of the potential risk should the virus mutate.

"This is a 'novel' (non-human) virus and therefore has the potential to cause a pandemic if it were to change to become easily and sustainably spread from person-to-person," it said in a statement, adding that has not happened.

The US government on Friday advised American citizens living in China of the cases but said no travel or trade restrictions would be applied to the country based on the current situation.

In Shanghai, residents were taking no chances, turning to traditional medicine and donning face masks.

Drugstores were running short of banlangen, a traditional Chinese medicine for colds made from the roots of the woad plant, used as a blue dye from ancient times.

"No one knows what might happen with bird flu, so they are buying it," said a clerk at the Ren Shou Tang medicine store.

The United Nations on Friday drew up a list of recommendations to try to curb the spread of H7N9 including regular hand washing, keeping animals away from living areas and avoiding eating sick animals.