Washington: A US government scientist cleaning out a storage room at a laboratory of the National Institutes of Health in Bethesda, Maryland, has found decades-old vials of smallpox, the second incident involving the mishandling of a highly dangerous pathogen by a US health agency in a month.

This is the first time that the deadly virus has been discovered outside the only two facilities in the world where smallpox samples are allowed, by international agreement, to be stored - a highly secure lab in Atlanta, US, and a virology and biotechnology research centre in Novosibirsk, Russia. It remains a mystery how the smallpox came to be stored at the laboratory.

A vial of smallpox vaccine. The virus was declared eradicated in 1980. Credit:Getty Images

The vials, which appear to date from the 1950s, were flown on Sunday night by government plane to the Centres for Disease Control and Prevention headquarters in Atlanta, Georgia, officials said on Tuesday. Initial testing confirmed the presence of smallpox virus DNA. Further testing, which could take up to two weeks, will determine whether the material is live. The samples will be destroyed after the testing is completed.

There is no evidence that any of the vials had been breached or that workers in the lab, which has been used by the Food and Drug Administration for decades, were exposed to infection. The Federal Bureau of Investigation and the CDC's division of select agents and toxins are investigating.