Withholding samples of the H5N1 bird flu virus jeopardizes the world's ability to protect itself against a potential pandemic, according to the head of the World Health Organization.

"If you do not share the virus with us, I want to be absolutely honest with you, I will fail you," Dr. Margaret Chan said in a speech to WHO's annual meeting of 193 member states in Geneva.

"I will fail you because you are tying my hands, you are muffling my ears, you are blinding my eyes."

Scientists at the agency's network of collaborative laboratories are studying samples of the H5N1 virus that has killed 185 people since 2003. Researchers are looking for any signs the disease, largely found in birds, has mutated into a pandemic-triggering form that can spread easily between people, or has become resistant to drugs.

China and Indonesia have held back samples of the virus. Last month, Chinese health officials said they will provide recent samples to WHO, but the agency is unsure when they will be sent, said WHO spokesman Gregory Hartl.

On Tuesday, Indonesian Health Minister Siti Fadillah Supari told the meeting that Jakarta has shared three viruses, but whether future samples will be shared is unknown. Supari complained poor countries provide samples that are used to develop commercial vaccines, and called for a fairer deal. 

"WHO recognizes the concern of many developing countries and I am fully behind you," Chan said Wednesday. "That's why we are taking a series of actions to make sure that developing countries have equitable access to affordable pandemic vaccines."

Up-to-date virus samples are needed to check if global stockpiles of antiviral medications and current vaccines will work against a pandemic strain, Chan said, calling a human flu pandemic the greatest threat to global health in the 21st century.

Experts at the meeting are working on a new formula for sharing samples, and updating regulations on infectious disease threats such as flu.

With files from the Associated Press