* Total of 28 cases in outbreak of new bird flu strain
* WHO investigating two suspected family clusters of infection
Another person died in China from a new strain of bird flu on Tuesday, state media said, raising to eight the number of deaths from the H7N9 virus since it was confirmed in humans for the first time last month.
The 83-year-old victim, from the eastern province of Jiangsu, was admitted to hospital with a fever on March 20 and confirmed as having H7N9 on April 2, Xinhua news agency said.
The H7N9 strain has now infected 28 people, all of them in eastern China, of whom eight have died. The cases include another four people confirmed to be infected with the virus, two in Shanghai and two in Zhejiang province, one of whom was dangerously ill, according to Xinhua.
The World Health Organization (WHO) said on Tuesday that it was looking into two suspected "family clusters" of people in China who may be infected with the H7N9 virus, potentially the first evidence of human-to-human spread.
The new virus is severe in most humans, leading to fears that if it becomes easily transmissible, it could cause a deadly influenza pandemic.
"At this point there is no evidence of sustained human-to- human transmission. There are some suspected but not yet confirmed cases of perhaps very limited transmission between close family members. Those are still being investigated," WHO spokesman Gregory Hartl told a news briefing in Geneva.
"In general, no matter what their exposure, this virus so far has produced overwhelmingly severe cases," he said.
Hartl said that the two suspected "clusters" are in Jiangsu and Shanghai, two of the four affected provinces. In each suspected cluster, a family member has been confirmed as being infected, the so-called "index case", he said.
"They are small family clusters. One is two people, and the other is three people. We just are trying to get good samples in one of the cases of the family cluster in order to be able to test those samples, and we are waiting test results from the other cluster."
The exact source of infection remained unknown, although samples had tested positive in some birds in poultry markets that remain the focus of investigations by China and the U.N.'s Food and Agriculture Organisation (FAO).
"If it is confirmed, it means that it has passed between two people under the same roof. And if it passes between two people under the same roof, it's either human-to-human transmission between two people who have constant and close contact or it could be virus from the same environmental source, so we don't know yet," Hartl said.
The WHO has praised China for mobilising resources nationwide to combat the strain by culling thousands of birds and monitoring hundreds of people close to those infected.
Chinese authorities have said there is no evidence of the H7N9 strain being transmitted between humans.
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 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 infected worldwide.
Chinese authorities initially tried to cover up the SARS outbreak.
"The Chinese ministry of health and family planning is working extremely openly and closely with us on this issue (H7N9). We are getting all the information that we need or ask for," Hartl said on Tuesday.
Referring to work being done in WHO laboratories worldwide to identify the best vaccine virus, he said: "In terms of vaccine development, we are still in the phase where a suitable vaccine virus is being developed. It is a matter of weeks not months."