The H7N9 strain of avian influenza spreads more easily from birds to humans than the previous H5N1 strain and is one of the "most lethal" flu viruses ever identified.

Keiji Fukuda, an influenza expert at the World Health Organisation, said the H7N9 virus appears to move from birds to humans with less difficulty than the previous strain, although scientists are still perplexed at how exactly the disease is spreading.

The H5N1 strain of bird flu killed 371 people worldwide between 2003 and 2013. Tens of millions of birds also died from the virus, and hundreds of millions were slaughtered to limit the spread of the disease.

WHO scientists are currently investigating and monitoring the H7N9 virus to establish whether it could spark a global pandemic, but say there is still no evidence to suggest it spreads from human to human.

However, experts are concerned about the disease's ability to jump to humans and how it can infect birds without causing noticeable symptoms, meaning it is difficult to monitor its spread.

Fukuda said: "This is definitely one of the most lethal influenza viruses we have seen so far."

Bird flu
An eight-year-old girl fought off the strain of H7N9 (Reuters)

At present, 108 people have been infected and 22 have died from the disease. Most cases have been reported near Shanghai but the infection is spreading.

A case of H7N9 has now been confirmed in the Shandong province, which is over 500 miles north of Shanghai. One case has also been reported in Beijing, north of Shandong.

The virus has also been found south of Shanghai in the Zhejiang province.

Fukuda said the WHO is concerned there may be many more people who are infected with the disease but are currently experiencing only mild symptoms.

Experts also said that while they are not sure of how the infection spreads, they have been encouraged by reports that the infection rate has slowed after live poultry markets have been shut down.

"Investigations into the possible sources of infection and reservoirs of the virus are ongoing. Until the source of infection has been identified, it is expected that there will be further cases of human infection with the virus in China," the WHO said.