Health experts have destroyed two-thirds of the specimens of a killer influenza virus sent as part of routine test kits around the world, but were still trying to trace two shipments that were supposed to go to Mexico and Lebanon, U.N. officials said Friday.

The World Health Organization (search) has been urging thousands of labs in 18 countries which received vials of the nearly 50-year-old H2N2 virus (search) to destroy the samples amid fears of a global pandemic should the virus be released.

WHO influenza chief Klaus Stohr said 10 of the countries which had received samples had confirmed their labs had destroyed the virus. Labs in Lebanon and Mexico, however, "never received the specimen even though they were on the distribution list," Stohr said.

He said it was possible the shipments had never been sent out at all, but he could not be sure. WHO has requested an investigation into the missing kits, he said.

Stohr said Hong Kong, Belgium, Singapore, Canada, Chile, France, Germany, Italy, South Korea and Taiwan had all confirmed their labs had destroyed their samples.

The five other countries with labs that received the kits were Saudi Arabia, Bermuda, Brazil, Israel and Japan. Stohr said Saudi Arabia had five labs with samples; four had already completed the destruction of the virus.

The other four countries had not yet sent confirmation but had received instructions to destroy the virus.

Most of the samples were sent starting last year at the request of the College of American Pathologists, or CAP, which helps labs do proficiency testing. The last shipments were sent out in February. It wasn't clear when the shipments were supposed to have gone to Mexico and Lebanon.

"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," Stohr said.

"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.