A top World Health Organization (search) official Friday warned that the avian flu virus is evolving quickly and urged heightened vigilance because the strain in China appears to have increased in virulence.

Shigeru Omi, the WHO's Western Pacific regional director, said two outbreaks in China's remote west in the past month have killed large numbers of species of birds that had previously been relatively resilient to the disease.

"The outbreaks indicate that the virus has become highly pathogenic to more and more species," he told a news conference.

"The virus remains unstable, unpredictable and very versatile," he said.

Omi said the H5N1 virus (search) is behaving differently in China and Vietnam. China has reported no human cases of bird flu; Vietnam has had 38 of Asia's 54 human deaths.

But the cases in Vietnam appear to be becoming less deadly, with fatality rates dropping from 60 to 70 percent last year, to about 10 to 20 percent in 2005, Omi said. Vietnam has also seen more cases where people are infected but don't develop symptoms.

"Anything could happen," he said. "Judging from the way the virus has behaved, it may have new and unpleasant surprises in store for us."

The only way to safeguard against further outbreaks or worse — a mutation of the virus into a form easily passed between humans — is "heightened vigilance," he said.

"Our work remains urgent," Omi said.

That means it's imperative for countries to share information, research and samples from their outbreaks with international agencies to strengthen efforts to fight the disease, he said.

Countries also need to strengthen their preparations, which include improved vaccine development and making anti-viral drugs available, Omi said.

Beijing has in the past been criticized for its reluctance to release information on its outbreaks.

Omi said the WHO is looking forward to getting samples from China because "sharing samples is very critical."

WHO officials said they were waiting for Beijing to approve a trip with Chinese health officials to Qinghai province, where more than 1,000 wild birds, including geese and gulls, were killed by the H5N1 flu strain in late May.

Gao Qiang, China's health minister, told reporters in Hong Kong that WHO teams would be free to visit infected areas.

"During the 2003 SARS outbreak, WHO experts were able to go wherever they wanted. And with bird flu, there's no place they can't go," Gao said.

On Thursday, the Agriculture Ministry confirmed a second outbreak of bird flu in the Xinjiang region.

It said it had culled more than 13,000 geese at a farm after discovering that about 460 had died from the virus.

China said both outbreaks appeared to be isolated incidents, with the case in Qinghai possibly carried there by migrating birds.