As China slaughtered millions of birds and banned livestock sales in big cities to prevent a human outbreak of bird flu, the World Health Organization China office urged the world's most populous country to intensify protection against possible human infection and the sharing of information and virus samples with other countries.

Roy Wadia, a WHO spokesman in Beijing, said it is difficult and even impossible to find out how many people might get infected in many Asian countries as a lot of farmers live side by side with their livestock.

He said the fact that no human infection cases have been reported doesn't mean there are no such cases.

Praising China for the huge resources it has put into its anti-bird flu campaign, he said it is "just a start", calling on China to stay vigilant against human infection.

China has experienced bird flu outbreaks in Anhui, Hunan, and Liaoning provinces and the Inner Mongolia Autonomous Region in recent weeks that have killed thousands of birds, but only reported three suspected human cases in central Hunan Province, currently diagnosed as "pneumonia of an unknown cause".

The possibility of human infection of H5N1 avian influenza has not been ruled out as all three ill persons had close contact withdead poultry. A 12-year-old girl, one of the three, died on Oct. 17.

Vietnam has suffered over 40 of the at least 62 bird flu human deaths in Asia since 2003.

South-east and East-Asia have large populations whose livestock stay in backyard farms or homes.

Facing with the WHO's warning of a possible human pandemic, China has toughened its measures to fight the highly pathogenic virus. The State Council--central government-- has launched a national command center for the prevention and control of bird fluand earmarked 250 million dollars to fund the battle.

Beijing also has announced to shut down all live poultry markets and Shanghai banned sales of live ducks, quail and other birds.

Health experts fear the bird flu could mutate into a form easily transmissible between humans, unleashing a pandemic that could kill millions.

A meeting of hundreds of international experts opened Monday in Geneva warned that a global human flu pandemic could cost the global economy at least 800 billion dollars.

To win the combat, Wadia said the WTO thinks it is vital for the countries with outbreaks to share information and samples of the most current virus strains to create effective vaccines.

"It's collaborative efforts and depend on every country's cooperation. If countries don't share, vaccines may not be complete. It will be impossible to protect humans without all virus strain combinations."

He urged China, as it did last year, to share virus samples of recent outbreaks with the international community .

A groups of WHO experts is expected to come to China at the invitation of the Chinese government to investigate the Hunan cases this week. A dialogue is underway between China and the WHO to determine what the assistance should be.

The WHO official suggested China's agricultural ministry should improve its bird-flu policy.