China on Tuesday said the H7N9 bird flu had spread to a new area as it confirmed the first case in the eastern province of Shandong in an outbreak which has so far killed 22 people.

Since China announced on March 31 that the virus had been discovered in humans for the first time, most cases have been confined to the commercial hub Shanghai and three nearby provinces, Jiangsu, Zhejiang and Anhui.

Beijing and the central province of Henan have also reported cases.

The health ministry said a 36-year-old man living in Shandong's Zaozhuang city was confirmed to have the virus, according to a statement on its website.

That case and three other new ones bring the total number of confirmed infections to 108, according to official figures.

Experts fear the prospect of such a virus mutating into a form easily transmissible between humans, which could then have the potential to trigger a pandemic.

But the World Health Organization (WHO) said Monday there was still no evidence H7N9 was spreading in a "sustained" way between people in China, though it was possible some family members may have infected one another.

"Right now we do not see evidence of sustained human-to-human transmission", said Keiji Fukuda, a top WHO influenza expert in a team visiting China to study H7N9.

Health experts differentiate between "sustained" human-to-human transmission and limited transmission, in which family members or medical personnel caring for the ill become infected.

Chinese health officials have acknowledged so-called "family clusters", where members of a single family have become infected, but have so far declined to put it down to human-to-human transmission.

The nine close contacts of the Shandong man were under medical observation, but so far were normal, the health ministry said.