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Chinese authorities said on Tuesday that the H7N9 bird flu has spread to a new part of the country after it was confirmed that a man in the northeastern province of Shandong has been infected by a new strain of bird flu.

The latest case brings the total number of those infected to 105, in an outbreak that has so far claimed the lives of 21 people.

While it is still not clear how people are becoming infected, experts at the Geneva-based World Health Organization (WHO) say there is no evidence that there is "sustained" transmission between people, according to Reuters.

"Investigations into the possible sources of infection and reservoirs of the virus are ongoing," WHO said on Monday. "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."

Since China first announced the discovery of the virus in humans on March 31, most bird flu cases have been found in Shanghai and three nearby provinces Jiangsu, Zhejiang and Anhui, with some cases also being reported in Beijing and the central province of Henan, according to AFP.

According to Xinhua, a 36-year-old man living in Shandong's Zaozhuang city was confirmed to have the virus on Monday.  He is now being treated in hospital, bringing the total number of confirmed cases to 105.

The nine people in close contact with the victim in Shangdong have been put under medical observation.  However, doctors say they are normal and are showing no signs of the infection, according to Xinhua.

Keiji Fukuda, WHO's assistant director-general for health, security and the environment said that researchers are now carrying out investigations in Shanghai, where many of the cases have occurred.

"Right now we are in the middle of our work. We have not come up with any final conclusions, and I think it is too early to say," said Fukuda, according to Reuters.

Fukuda also said that so far there is no evidence of  "sustained human-to-human transmission," according to AFP.

Experts say "sustained" human-to-human transmission is different from limited transmission, in which family members or medical personnel treating the ill become infected.

While health officials in China have found evidence of so-called "family clusters", where members of a single family have become infected, they have so far declined to put the cases down to human-to-human transmission.

Other bird flu strains like H5N1, which has been circulating for many years and can be passed from bird to bird and bird to human, does not generally pass from human to human.

However, Ho Pak-leung, an associate professor in the Department of Microbiology at the University of Hong Kong, said in the British Medical Journal (BMJ) that in the two months since H7N9 was first detected, the new strain has already infected nearly twice as many people in China as H5N1 did in a decade, according to Reuters.

"H7N9 is much more transmissible to humans, and it's much more difficult to track down," the professor told BMJ. "We don't understand why it's so difficult to find."

Ho believes that the difficulties may be linked to the sampling techniques or testing levels.

"It (also) might be that they are not sampling enough animal species, and they may have to take a look at the less common species of birds being sold in Chinese markets," he added.