China has shared long-sought samples of the H5N1 bird flu virus, in what many scientists view as a breakthrough in cooperation, the World Health Organisation (WHO) said on Thursday.

The samples, taken from some of the thousands of wild birds which died in Qinghai Lake in April 2005, have been sent to the U.S. Centers for Disease Control (CDC), a WHO collaborating centre in Atlanta, for further analysis, WHO officials said.

"My understanding is they have been shipped to a WHO collaborating centre now from the Ministry of Agriculture," WHO scientist Michael Perdue told a news briefing.

He said the animal samples, the first from China in 2-1/2 years, should help scientists understand the origin of a sub-type of the deadly H5N1 virus which later circulated in Turkey and Africa but is genetically different from the one hitting other parts of Asia, including Indonesia, Thailand and Vietnam.

Leading scientists have repeatedly expressed concerns that China was not sharing all genetic data from bird flu cases -- widely seen as vital to track mutations and develop a vaccine against a human pandemic.

"Most people do see it as a breakthrough for cooperation," Perdue told Reuters.

The lake, a major stop-off point for migratory birds in the remote northwestern Qinghai province, has become a focus for scientists studying mutations of the virus.

The Agriculture Ministry previously had placed "certain restrictions" on sharing the virus samples, but this had been resolved through negotiations, Perdue said without giving details.

He said the samples from birds which died at the lake were shipped a week ago from a key Ministry of Agriculture laboratory in Harbin to the CDC.

CDC officials expected to receive the samples later on Thursday and it was hoped they would include additional animal samples requested from China, he said.

"We have to wait to see exactly what they shipped," he said.

Chinese scientists have published some analyses of the samples, but the CDC is expected to carry out more sophisticated testing with antibodies which will reveal variations of immune responses, according to the WHO expert.

"It gives us an animal index case for birds to have the samples from Qinghai Lake, which is the first time that particular type was detected in China," Perdue told Reuters.

"It will go into the global collection we are making of the virus to see how it changes over time," the American added.

Hualen Chen, head of China's national avian influenza reference laboratory in Harbin, was among 70 scientists worldwide who last month announced the launch of a global body to share genetic bird flu data.

China has shared samples from human cases, David Heymann, WHO's acting special representative on avian influenza, said.

The WHO has confirmed 21 human bird flu cases including 14 deaths in China since November 2005.

Bird flu remains essentially an animal disease, but has killed 148 people of 251 people confirmed as having the virus in 10 countries since 2003, according to the Geneva-based WHO.