China's decision came just one day after its candidate, Hong Kong-born Margaret Chan Fung Fu-chun, was elected director-general of the World Health Organization, and only hours after she told journalists in Geneva she would definitely speak out and urge China to share information.

The WHO said 20 virus samples were being sent to the US Centers for Disease Control and Prevention, a WHO collaborating center, raising hopes of a better understanding of how the H5N1 bird flu virus is changing.

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"In our cooperation, we can have had some misunderstandings, but generally it is very good and normal," Jia Youling, the Chinese chief veterinary officer at the Agriculture Ministry told a press conference Friday.

He insisted the ministry had met obligations laid down at the end of last year to share samples of virus strains with the WHO so they could be sent for further testing.

Jia said there had been a delay in sending the samples, which he ascribed to Beijing's scrupulous attention to safety regulations.

"I want the journalists to understand that those viruses are highly virulent," he said. "It is difficult to transport them. If anything happens during the course of the transportation, the consequences would be very serious."

He said the central government intended to step up its cooperation with international health organizations.

"The Ministry of Agriculture will step up cooperation with the Food and Agriculture Organization and the World Organization of Animal Health, and will increase cooperation with the WHO in active collaboration with the Ministry of Health," Jia said.

Henk Bekedam, the WHO's China representative, said: "We are very encouraged by that. They are viruses from 2004 and 2005, and we will make follow-ups for the 2006 samples."

The decision comes after Beijing rejected findings in a paper published last week by Hong Kong and United States scientists which said they had detected a new strain of H5N1 in Fujian province last year.

"The data cited in the article was unauthentic, and the research methodology was not based on science," Jia said.

The paper, published in the US journal Proceedings of the National Academy of Sciences, that identified the "Fujian strain" said it may have started outbreaks in Southeast Asia.

"In fact, there is no such thing as a new `Fujian-like' virus variant at all," Jia said.

"It is groundless to assert that the outbreak of bird flu in Southeast Asian countries was caused by avian influenza in China and there would be a new outbreak wave in the world."

The WHO, which says it is still studying the paper, said its understanding had been hampered by China's refusal to share bird flu samples.

But Jia said when China cooperated in the past, the samples had been misused.

"Four of the five virus samples we provided were used by foreign research institutes in an inappropriate way twice, infringing intellectual property rights of Chinese research institutes," he said.

The WHO's Bekedam said the viruses were used in research that did not acknowledge it was China's Ministry of Agriculture that identified the virus, in breach of scientific protocol.

"That happened twice, and I apologized on behalf of the WHO collaborating center because that is bad behavior among scientists," he said.

Mainland scientists also denied the paper's claims that its vaccines were ineffective against new strains, saying they were continually updating vaccines as the virus changed.

"We have developed new vaccines to control those variants," said Chen Hualan, director of China's National Avian Influenza Reference Laboratory.

China's national bird flu laboratory had been instructed by the Agriculture Ministry to follow any signs of mutation, amid fears the virus could change into a form that can pass easily between people, potentially leading to a pandemic, Jia added.

He also attacked the methodology and ethics of Guan Yi, one of the paper's authors. Guan, who heads the emerging infectious diseases lab at Hong Kong University, Friday declined to comment. But earlier he said: "I stand by those findings."

Jia said the Fujian-like strain, which Guan said had emerged in March 2005, was actually the same as bird flu viruses found in Hunan in February 2004 in terms of genetic sequencing.

"Guan said he wanted to alert the world with the paper, but why didn't he report the markets with virus-carrying birds to the government if he truly believed in his findings?" Jia asked.

He said there were 10 confirmed poultry outbreaks in seven mainland provinces this year, adding that 95 percent of domestic birds had been vaccinated.

The WHO has said the Fujian strain has not shown a heightened danger to humans.

H5N1 has caused 21 human infections in the mainland since late 2003, including 14 deaths. With the world's largest poultry population and millions of backyard birds, the mainland is seen as crucial in the fight against bird flu.