BEIJING (Reuters) - The World Health Organisation urged vigilance against a deadly strain of bird flu on Friday, warning that the disease scientists say could cause a global pandemic was moving in new and unpredictable ways.

Shigeru Omi, the WHO´s Asia director, made the comments in Beijing a day after China said it had discovered the H5N1 strain of bird flu in the far western province of Xinjiang, its second outbreak in as many months.

"All of this shows the virus remains unstable, unpredictable and very versatile," Omi told a news conference."It may have new and unpleasant surprises in store for all of us."

Avian flu is highly infectious in birds and does not spread easily in humans, but scientists fear it could mutate into a form better able to pass from animals to people, possibly triggering a pandemic that they say would likely start in Asia.

H5N1 first surfaced in poultry in Hong Kong and China eight years ago and has killed at least 37 people in Vietnam, 12 in Thailand and four in Cambodia.

Omi said the virus appeared to be moving in different ways in different places, apparently becoming more transmissable but less fatal in Vietnam while becoming more pathogenic in China.

"It is very difficult to predict what will happen under these circumstances, but the best thing I can say is to keep our vigilance high," he said.

There were at least two cases, in Thailand and Vietnam, where the disease was likely to have passed between humans, Omi said, but added "there is no evidence for effective human to human transmission yet".

Sharing information, making virus samples available to WHO laboratories and improving animal husbandry practices were all key to preventing a pandemic, Omi said.

But the disease usually appears in rural areas where surveillance is harder and involves both health and agriculture sectors, making a coordinated response a challenge.

The World Health Organisation was still awaiting a response from China on its proposed joint mission to affected sites, but said it was satisfied with Beijing´s response to the outbreaks.

"The China authorities are very serious about avian influenza. Their response has been very strong," WHO representative Hank Bekedam said.

China said the outbreak in domestic poultry in a far northwestern part of Xinjiang had been brought under control through a combination of culling, quarantines and vaccination.

The government "ordered all the poultry markets in Tacheng City to be closed down and had 17,014 birds slaughtered in the affected area, most of them chickens and geese," the official Xinhua news agency reported.

"Roads to the affected area have been blocked and traffic has been closed," it said.

Last month China reported an outbreak in wild birds in the northwestern province of Qinghai, prompting an extensive vaccination campaign.