China confirmed three more cases of bird flu on Sunday, as authorities sought to curb a deadly outbreak by banning live poultry sales and disinfecting schools.

The new cases of the H7N9 strain of avian influenza brought the number recorded in China's developed eastern region to 21 since it was announced a week ago that the virus had been found in humans for the first time.

Six people have died, including four in the commercial city of Shanghai, which has recorded 10 cases of the disease.

Of the three newly confirmed cases, two were elderly men in Shanghai who are receiving treatment, Shanghai's local government said.

The third was a 55-year-old man working in the live poultry trade and was reported in east China's Anhui province, according to state news agency Xinhua.

Taiwan's Centers for Disease Control said Sunday it was investigating two possible cases involving people who travelled to the affected area in mainland China.

China stepped up efforts Sunday to halt the deadly outbreak, with state media saying "intense" farming methods had heightened the risk of deadly diseases crossing from animals to humans.

The education ministry ordered schools nationwide to "guarantee" the health of students against infectious disease, including H7N9, by ensuring the safety of food and monitoring for symptoms of illness.

In Shanghai workers sprayed disinfectant in classrooms, local television showed, while markets were closed to halt the spread of the disease.

Shanghai had already banned live poultry trading while nearby Nanjing city has followed suit and Hangzhou culled poultry after discovering infected quail.

Dead sparrows found in Nanjing, which sparked alarm after photos were posted online, had tested negative for H7N9, state media said, providing some relief.

In an editorial on Sunday the state-backed Global Times newspaper said farming methods were helping to spread the disease.

"In China's southern and eastern coastal areas, agriculture, especially animal husbandry, has become more intense and populations more dense," said the English-language edition of the paper.

"There is greater chance of contact between humans and animals and subsequent diseases."

It called for higher standards in the agricultural industry and more balanced development, instead of a narrow focus on rapid economic growth.

Chinese authorities, who have confirmed H7N9 in birds, maintain there is no evidence of human-to-human transmission of the virus.

"Currently, there is still no evidence showing the H7N9 bird flu virus is transmitted between people," Shanghai health official Wu Fan told an online chat on Sunday.

The World Health Organization has played down fears over the H7N9 strain, agreeing there was no evidence of human-to-human transmission, but saying it was crucial to find out how the virus infects humans.

Like the more common H5N1 variant which typically spreads from birds to humans through direct contact, experts fear such viruses could mutate into a form easily transmissible between humans, with the potential to trigger a pandemic.

China has been hit by a series of food scandals in recent years, some caused by producers deliberately using sub-standard or illegal ingredients, despite government vows to crack down on malpractice.

Last month more than 16,000 dead pigs were found floating in a Shanghai river after being discarded by farmers upstream, casting a spotlight on China's poorly regulated agricultural industry.

Some of China's 500 million Internet users expressed worry over H7N9, despite government assurances.

"Ten years ago there was SARS, ten years later there is bird flu... Ten years later the experts are still talking nonsense," said a microblog posting under the name Heshang Kaifang Nanigu.

China has sought to improve transparency surrounding H7N9, state media says, after being accused of covering up the 2003 outbreak of Severe Acute Respiratory Syndrome (SARS), which killed about 800 people globally.