BEIJING, China (AP) -- China ordered the closure of all live poultry markets in Beijing and conducted door-to-door searches for chickens and ducks as it toughened its efforts to fight bird flu, and the World Health Organization warned that a global human flu pandemic is inevitable.Beijing also announced that 6 million birds had been slaughtered around the site of China's most recent bird flu outbreak, and WHO said it had been asked to help in the reopened investigation of the country's possible first human cases of the virus.The escalation of anti-bird flu measures in the world's most populous country came as a meeting of hundreds of international experts in Geneva opened Monday with warnings that a global human flu pandemic could cost the global economy at least US$800 billion (euro675 billion). (Full story)Experts fear the bird flu virus that is sweeping through Asia and has entered Europe could mutate into a form that is easily passed between humans, producing a pandemic that could kill millions.The virulent H5N1 strain has killed at least 62 people in Asia since 2003, and resulted in the death or destruction of millions of birds.Beijing on Sunday reopened an investigation into whether bird flu killed a 12-year-old girl and sickened two people last month in cases originally ruled not to be H5N1.Roy Wadia, a WHO spokesman in Beijing, said on Monday discussions were under way with Chinese officials about what role the agency could play in the investigation, and a decision was likely within days.China has had no confirmed human infections. But it has imposed increasingly strict measures following warnings that a human case was inevitable if China can't prevent outbreaks among its 5.2 billion chickens, ducks and other poultry.Experts are especially worried about China because of the vast scale of its poultry industry and because major migration routes for wild birds pass over it.After China's latest outbreak, in Liaoning province, east of Beijing, authorities destroyed 6 million poultry in 15 villages near the site, where the disease killed 8,940 chickens, the Xinhua News Agency said on Monday.Authorities closed all of Beijing's 168 live poultry markets as a precaution against the possible spread of the virus in the city, state television reported.In Shanghai, China's largest city, sales of live ducks, quail and other birds have been banned, officials said.In Vietnam, a leading European health official conceded that the European Union should have acted earlier to help Asian nations fight bird flu and pledged euro30 million (US$35.7 million) to help the region combat the virus.Also Monday, Swiss drug maker Roche said it would increase production of the antiviral Tamiflu to make 300 million treatments by 2007 to try to cope with international demand.At the Geneva meeting, the World Bank's lead economist for East Asia and the Pacific, Milan Brahmbhatt, warned that "panic and disruption" caused by a global human flu pandemic could cause world gross domestic product to drop by 2 percent or more -- amounting to about US$800 billion (euro675 billion) over one year.Meanwhile, authorities in Japan detected signs of a bird flu outbreak at another poultry farm close to a recently affected ranch near Tokyo, the Agricultural Ministry said Monday. Tests showed the chickens had antibodies for a virus from the H5 family -- meaning that they were infected in the past but had survived -- at Moriya Farm in the town of Ogawa in Ibaraki prefecture (state), ministry official Akiko Suzuki said.In Thailand, health authorities on Monday announced increased surveillance of patients with possible cases of bird flu to improve their chances of survival.Copyright 2005 The Associated Press. All rights reserved.This material may not be published, broadcast, rewritten, or redistributed.