Health experts have destroyed most samples of a deadly influenza strain mistakenly sent to labs around the world; but two shipments meant to reach Mexico and Lebanon are missing, UN officials said Friday.

"We don't know where these boxes got lost, but the investigation into what has happened between the shipment of these panels and their non-arrival is ranking very high on our 'to do' list," WHO influenza chief Klaus Stohr said, referring to the Mexico and Lebanon shipments.

The samples were unintentionally sent to nearly 4,000 labs in 18 countries at the request of the College of American Pathologists, which assists laboratories to do quality testing.

Two-thirds of them have been destroyed so far, The World Health Organization confirmed Friday, but two shipments meant to reach Mexico and Lebanon are unaccounted for.

Stohr said Friday that 10 countries that had received samples confirmed their labs destroyed the virus. Those countries include: Canada, Chile, France, Hong Kong, Belgium, Germany Italy, South Korea, Taiwan and Singapore.

However, laboratories in Lebanon and Mexico "never received the specimen even though they were on the distribution list," Stohr said.

He said it was possible the samples had never been sent to the two countries, but that he couldn't be sure.

The five other nations that had received the samples were Saudi Arabia, Bermuda, Brazil, Israel and Japan.

Stohr said four of the five labs in Saudi Arabia that received the samples had destroyed them. The other four countries had not yet confirmed that they followed up on instructions to destroy the samples.

The UN health agency officials urged laboratories to destroy the kits after first being alerted of their existence by Canadians who found the vials in their British Columbia laboratory.

They reported their findings to the Public Health Agency of Canada and the World Health Organization was subsequently warned on April 8.

The Canadian laboratory received the samples in February, but officials were not sure when they should have been sent to Mexico and Lebanon.

"We are worried, but CAP said there is a possibility they were never sent. (Otherwise), I cannot say at this stage what we would possibly do," Stohr said.

"The carrier, the transporter and packager would have to be questioned particularly about these packages.

The samples contain the deadly H2N2 virus, otherwise known as the "Asian flu".

H2N2 caused the 1957 pandemic that killed an estimated one million to four million people around the world. It was last seen in humans in 1968.

With files from The Associated Press