BEIJING, April 20 (Reuters) - China has agreed to send human bird flu samples to the World Health Organisation, state media said, in an apparent response to complaints that its reluctance to share the samples was frustrating 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.

"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," the China Daily on Friday quoted a Ministry of Health official surnamed Ma as saying.

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 transmissable among people.

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

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

In China"e;s case, the contention is not so much about drug access as the country has homegrown laboratories researching their own H5N1 vaccines. Chinese scientists, like their counterparts elsewhere, are possessive about their samples.

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

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"e;s largest poultry population and millions of backyard birds, is seen as key in the fight against bird flu.