(Recasts with WHO comment, adjusts slug from earlier BIRDFLU-CHINA/)

BEIJING, April 20 (Reuters) - China has agreed to share human bird flu samples, the World Health Organisation said on Friday, following complaints that its reluctance to send the samples could frustrate global efforts to fight the virus.

China last shared human samples of the H5N1 virus with WHO collaborating laboratories a year ago, but it has since reported five more human infections and the WHO has repeatedly asked for new samples.

"This week, the Ministry of Health informally told us that they'll share the viruses that we requested," said Joanna Brent, a WHO spokeswoman in Beijing.

"Normally when we hear those kinds of reassurances, we're pretty positive that they will be sharing shortly," she said.

Scientists stress the importance of sharing samples of viruses, which are constantly changing, to see if they have developed resistence to drugs or become more transmissible among people.

But those calls are only sometimes observed and the WHO is not empowered to oblige any government to share.

Indonesia and Thailand have protested that drug companies were using samples from developing countries to make expensive vaccines that these same nations would be unable to afford.

China has complained that samples it sent to WHO-affiliated laboratories in 2004 were used in research that failed to acknowledge the contribution of Chinese scientists, in a breach of protocol for which the WHO has apologised.

The state-run China Daily quoted a Ministry of Health official as saying it was preparing to share new samples.

"As requested by the WHO, we will send two recent samples of the virus and one from a Beijing patient who was infected in 2003," it quoted a ministry official surnamed Ma as saying. official surnamed Ma as saying.

Another official, Wang Lin, a press officer at the Chinese Centre for Disease Control and Prevention, was quoted as saying the handover of samples was a long process because of the extreme caution with which dangerous viruses are handled.

But Brent said the WHO expected the promised samples soon.

"Organising logistics can be quite complex, so at this stage we don't have a timeline. But we feel rather hopeful that it will be in the near future," she said.

While bird flu is mainly a disease in animals, experts fear it could mutate into a form that can be passed easily among people, triggering a possible pandemic.

The virus has killed 170 people worldwide since late 2003, and China, with the world's largest poultry population, millions of backyard birds, and a strained medical system, is seen as key in the fight against bird flu.

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