The World Health Organization (WHO) has obtained only six human samples of the virus and no infected poultry samples in eight months.

Although they are supposed to be working together, Michael Perdue, of the WHO's flu program, told Nature that the UN Food and Agricultural Organization (FAO) "hasn't been sharing" the samples it has.

"It's as if you hear a noise in your car engine but you keep driving, not knowing whether it's serious," said Klaus Stohr, co- ordinator working for the WHO flu program.

Studying samples of the H5N1 bird flu virus that has killed more than 50 people in Asia since 2003 and tracking genetic changes is essential, scientists say.

They fear the virus could mutate and develop into a worldwide pandemic with the potential to kill millions of people.

The journal said the FAO should be collecting samples but it has not been receiving any, so the WHO does not know how the virus is changing.

Joseph Domenech, of the FAO headquarters in Rome, said some infected countries do not have the resources to collect and export samples.

"But things that should be happening are not," he said. "Samples sometimes sit in labs."

Infected nations are also worried about losing control and negative publicity.

"They don't want outside groups making pronouncements and these getting into the press without being vetted by the ministries of health and agriculture," Domenech told Nature.

To overcome the problem, WHO representatives met government officials from Viet Nam, Cambodia and Laos to ask for poultry samples of the virus to be sent directly to it.

"The presentations drove home the importance and urgency of sharing data," Perdue told Nature.