China today revealed it had discovered the bird flu virus in pigs, a deadly disease that has killed 27 people in Asia this year.

The Chinese announcement - which said the H5N1 strain of avian influenza had been found in pigs as far back as last year - could have ominous implications for efforts to restrict the disease's spread to humans.

The delay in providing the information also appears to reflect the fact that secrecy has been a recurring problem in the fight against bird flu in Asia.

Deputy Prime Minister Najib Razak said Malaysia would reveal all facts about the outbreak in a transparent manner following an apparent attempt to muzzle the local press .

Meanwhile, the World Health Organisation (WHO) expressed support for Malaysia's response to the outbreak and the prompt manner it informed international organisations and neighbouring countries.

"Transparency such as Malaysia has displayed is vital in the fight against the virus," said Shigeru Omi, regional director for the western Pacific.

Long and difficult struggle

The WHO was "troubled" by the outbreak in Malaysia, which had implemented aggressive measures to prevent importation of the virus.

"H5N1 has once more shown how well it is entrenched in parts of Southeast Asia. It is going to be a long and difficult struggle to eliminate this virus from the environment. And the longer it takes, the greater the risk to public health," Omi said in a statement.

"As long as the virus circulates in poultry this way, there will be a risk of it jumping to humans, with the potential for a pandemic influenza strain," he said.

Scientists have warned that if pigs are infected the virus could mutate to a more lethal form that could spread more easily to humans because they are a notorious vector for unleashing flu pandemics, being a "mixing vessel" in which viruses swap genes.