"There's a slim but a real risk that this could spark a pandemic," Maria Cheng, a spokeswoman for the UN's health agency, said on Wednesday.
She said many people around the world would have no protection if the virus were released from the high-security labs.
The virus, known as H2N2, killed between one million to four million people worldwide during the Asian influenza pandemic of 1957-58 before disappearing in 1968.
"As far as pandemics go, it (the event in 1957-58) was relatively mild. But if this were to recur, it would have significant consequences for the public health system," Cheng told AFP.
Security experts fear that a resurgence of ancient or stored viruses such as smallpox or past flu strains could be used by bioterrorists.
A US-based institution, the College of American Pathologists (CAP), distributed the flu samples to 3747 laboratories run by the private Meridian Bioscience Inc of Cincinnati, Ohio.
The samples were to be tested as part of "routine quality-control certification".
People born after 1968 probably
have little immunity to the strain
The problem was first detected on 25 March by Canada's National Microbiology Laboratory, the WHO said.
The WHO warned in a statement on Tuesday that people born after 1968 would probably have no or only limited immunity to the strain, which is not contained in current influenza vaccines.
"As a precautionary measure, WHO is recommending that all samples of the proficiency testing panel from CAP and other proficiency testing providers containing H2N2 and any derivates be destroyed immediately," the agency said.
The US government on 8 April asked the college to instruct labs that had received the samples to destroy them.
A second message sent on 12 April "further requested that destruction of the H2N2 virus be confirmed and that any case of respiratory disease among laboratory workers be investigated and notified to national authorities", it added.
Cheng said 90% of the laboratories were in North America.
Sixty-one laboratories are in 16 countries in Asia, Europe, the Middle East and South America.
"If a laboratory accident were to occur, a person could become infected. If that happened, that person would likely fall ill and he or she could infect somebody else. And that could mark the beginning of a global outbreak," the WHO's top flu expert, Klaus Stohr, told The Washington Post.
"We are talking about a fully transmissible human influenza virus to which the majority of the population has no immunity. We are concerned."
The WHO said normally only circulating influenza A virus strains, which people had been exposed to in recent years, were sent out in proficiency testing kits.
US authorities and the US Centres for Disease Control and Prevention (CDC) in Atlanta were investigating the incident at CAP, Cheng said.
"The risk for the general population is also considered low"
World Health Organisation
So far, there have been no reports of infection among laboratory workers associated with the distribution of the H2N2 samples, the organisation said.
"While a few H2N2 laboratory-acquired infections have been documented in the past, the likelihood of laboratory-acquired influenza infection is considered low when proper bio-safety precautions are followed," the WHO said. "The risk for the general population is also considered low."
Laboratories handling flu virus samples normally should be rated at a high bio-safety level 2, which would also cover the H2N2 strain, according to Cheng.
Risk played down
In London, a spokeswoman for the National Institute for Medical Research also played down the risk of a contagion.
"It's been dealt with in the press in a very sensational way, but these samples have all been sent to category 3 and 4 labs, so the likelihood of anything popping off is small," she said.
The WHO called for a bio-safety review among all laboratories that store or handled past flu viruses.
Stored samples of viruses and bacteria are valuable for scientists. They serve as a historical comparison for newly emergent viral strains, a reference point in vaccine research and a vital source for researchers working on the flu's genetic code.