(Adds new death)
By Ben Blanchard
BEIJING, April 4 (Reuters) - China said it was mobilising resources nationwide to combat a new strain of deadly bird flu that has killed four people, as Japan and Hong Kong stepped up vigilance against the virus and Vietnam banned imports of Chinese poultry.
The H7N9 bird flu strain does not appear to be transmitted from human to human but authorities in Hong Kong raised a preliminary alert and said they were taking precautions at the airport.
In Japan, airports have put up posters at entry points warning all passengers from China to seek medical attention if they suspect they have bird flu.
A total of 11 people in China have been confirmed to have contracted H7N9, all in the east of the country.
Another person died on Thursday, in Shanghai, bringing the number of deaths to four, state media said. Xinhua news agency said the victim was a 48-year-old man who worked delivering poultry.
"(China) will strengthen its leadership in combating the virus ... and coordinate and deploy the entire nation's health system to combat the virus," the Health Ministry said in a statement late on Wednesday on its website (www.moh.gov.cn).
In Hong Kong, authorities activated the preliminary "Alert Response Level" under a preparedness plan for an influenza pandemic, which calls for close monitoring of chicken farms, vaccination, culling drills, and a suspension of imports of live birds from the mainland.
All passengers on flights in and out of Hong Kong were being asked to notify flight attendants or airport staff if they were feeling unwell.
Vietnam said it had banned poultry imports from China, citing the risk from H7N9.
In Beijing, the Health Ministry said the government would swiftly communicate details of the new strain to the outside world and its own people, following complaints it had been too slow to report on the outbreak and suspicion of a cover-up.
Chinese internet users and some 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. The government has said it needed time to correctly identify the virus.
In 2003, authorities initially tried to cover up an epidemic of Severe Acute Respiratory Syndrome (SARS), which emerged in China and killed about 10 percent of the 8,000 people it infected worldwide.
China "will continue to openly and transparently maintain communication and information channels with the World Health Organisation and relevant countries and regions, strengthen monitoring and preventative measures", the ministry said in a statement.
Flu experts across the world are studying samples isolated from the patients to assess the human pandemic potential of the strain.
Other strains of bird flu, such as H5N1, have been circulating for many years and can be transmitted from bird to bird, and bird to human, but not generally from human to human.
So far, this lack of human-to-human transmission also appears to be a feature of the H7N9 strain.
However, China has yet to find any animals infected with H7N9, meaning how humans got it remains a mystery.
"The gene sequences confirm that this is an avian virus, and that it is a low pathogenic form (meaning it is likely to cause mild disease in birds)," said Wendy Barclay, a flu virologist at Britain's Imperial College London.
"But what the sequences also reveal is that there are some mammalian adapting mutations in some of the genes."
This, she said, meant the H7N9 virus has already acquired some of the genetic changes it would need to mutate into a form that could be transmitted from person to person.
While the official Xinhua news agency said it was unfair to compare SARS with H7N9, as the new bird flu virus had yet to show signs of human-to-human transmission, it did warn that the government's credibility was on the line.
"If there is anything that SARS has taught China and its government, it's that one cannot be too careful or too honest when it comes to deadly pandemics. The last 10 years have taught the government a lot, but it is far from enough," it said in a commentary. (Additional reporting by Clare Baldwin in HONG KONG, Olivier Fabre in TOKYO, Hanoi newsroom and Kate Kelland in LONDON; Editing by Raju Gopalakrishnan and Robert Birsel)