A strain of bird flu that has been found in humans for the first time in eastern China is not a cause for panic, the World Health Organisation (WHO) has said, as the number of people infected rose to 21, including six deaths.

The WHO on Monday praised China for mobilising resources nationwide to combat the H7N9 flu strain by culling tens of thousands of birds and monitoring hundreds of people close to those infected.

"So far, we really only have sporadic cases of a rare disease, and perhaps it will remain that way. So this is not a time for over-reaction or panic," said Michael O'Leary, the WHO's representative to China.

Li Bin, the head of China's National Health and Family Planning Commission, said on Sunday she was confident authorities could contain the virus.

"These are a relatively small number of serious cases with personal health, medical implications, but not at this stage known public health implications," O'Leary told reporters at a joint press briefing with the Chinese government.

But he warned that information on the virus was still incomplete.

"We really can't rely on information from other viruses. H7N9 is a new virus in humans and the pattern that it follows cannot be predicted by the patterns that we have from other influenza viruses," O'Leary said.

No cases have yet been reported outside of China, he said.

In total, 621 close contacts of the 21 people known to have been infected are being closely monitored and have yet to show symptoms of infection, Liang Wannian, the director of China's H7N9 prevention and control office, said.

"In recent years, there have been huge changes in our national epidemic system, especially our health emergency response ability," Liang said.

The bird flu outbreak has caused global concern and some Chinese internet users and newspapers have questioned why it took so long for the government to announce the new cases, especially as two of the victims fell ill in February.

Airline shares have fallen in Europe and in Hong Kong over fears that the new virus could be lead to an epidemic like Severe Acute Respiratory Syndrome (SARS), which emerged in China in 2002 and killed about 10 percent of the 8,000 people it infected worldwide.

Chinese authorities initially tried to cover up the outbreak of SARS.

In the H7N9 case, it has said it needed time to correctly identify the virus, with cases spread between Shanghai and eastern Zhejiang, Jiangsu, and Anhui provinces.

Other strains of bird flu, such as H5N1, have been circulating for many years and can be transmitted from bird to bird, and bird to human, but not generally from human to human.