Four more people in China have died from a new strain of bird flu, bringing to 31 the number of deaths from the mysterious H7N9 virus, with the number of infections rising by two to 129, according to Chinese health authorities.

Among the deaths, two occurred in the eastern province of Jiangsu; one was from eastern Zhejiang; while another was from central Anhui, based on a Reuters analysis of the data provided by Chinese health authorities on Monday.

The government did not provide more details of the victims.

Chinese health authorities said two new infections were reported in the eastern coastal province of Fujian. The virus, which was mostly concentrated in the region around the commercial capital of Shanghai, spread to Fujian in late April.

The Geneva-based World Health Organization (WHO) has said it has no evidence that the new strain of bird flu, which was first detected in patients in China in March, is easily transmissible between humans.

Chinese scientists have confirmed that the H7N9 strain has been transmitted to humans from chickens. But the WHO has said 40 percent of people infected with H7N9 appear to have had no contact with poultry.

The head of the U.S. Centers for Disease Control and Prevention said the current strain of bird flu cannot spark a pandemic in its current form - but he added that there is no guarantee it will not mutate and cause a serious pandemic.