BANGKOK, Jan 29: The World Health Organization warned on Thursday that unsafe poultry culling to halt bird flu's spread may raise the risk of the virus mutating into a more deadly strain.

"If the killing of birds is done in such a way that exposes more people (to the virus), then this ... could be increasing the risk of developing a strain that you would not want to see," said WHO spokesman Dick Thompson.

The WHO has warned that while humans have so far only caught the disease through contact with infected birds or their droppings, it could claim millions of lives if it mutates into a form that can be passed among humans - a risk that rises with the more people who fall ill.

Countries across Asia have battled allegations of a cover-up, with China denying allegations Thursday that it was the source of the bird flu outbreak which has hit 10 Asian nations so far.

China became the latest Asian government to face charges of covering up the disease when the respected New Scientist journal in Britain said the epidemic probably erupted there as early as a year ago.

"A combination of official cover-up and questionable farming practices allowed it to turn into the epidemic now under way," the weekly said, citing unnamed health experts.

But as China halted poultry exports from its three affected provinces, Guangxi in the southwest near the Vietnam border and the central provinces of Hubei and Hunan, the foreign ministry rejected the charges.

WHO has asked for an explanation of the deaths last February of two Hong Kong tourists who had visited southern China, as part of efforts to track the origins of the outbreak which has so far claimed 10 lives.

However, the WHO's Beijing-based spokesman Roy Wadia said that despite the suspected link with the dead tourists, it was too early to confirm China as the origin of the disease which could have had several possible sources.

New Scientist said it suspected the H5N1 virus was disseminated through a faulty mass poultry vaccination by Chinese farmers anxious to protect their flocks from the disease which triggered a mass cull in Hong Kong in 1997.

Proof of a cover-up would be extremely embarrassing for China which was strongly criticised last year after it emerged that authorities had hidden the outbreak of Severe Acute Respiratory Syndrome (SARS) for months.

In Thailand, the government also faced heavy flak after it admitted it had "screwed up" in its handling of the bird flu crisis which has now erupted in nearly half its 76 provinces and killed two children.

Prime Minister Thaksin Shinawatra admitted mistakes had been made despite the lessons learned during the panic over SARS, which sprang from southern China and went on to claim nearly 800 lives with infections in 32 countries.-AFP