A gas explosion ripped through a Russian research laboratory that stores highly infectious viruses including smallpox, Ebola and HIV.
The blast sparked a fire at the site which is one of just two places in the world that holds smallpox, one of the most devastating diseases ever which has been declared eradicated.
A worker suffered third-degree burns in the explosion which occurred during repairs to a sanitary inspection room on the fifth floor at the State Research Centre of Virology and Biotechnology.
The Siberian facility, also known as Vector, housed the Soviet Union's biological weapons research during the Cold War before it was transformed into one of Russia's primary disease research centres.
A "major incident" was declared after a "gas cylinder explosion" blew out glass windows and sparked a fire that spread to 30 square metres before it was finally extinguished.
Russia insisted the parts of the building affected by the blast and fire did not hold any biohazardous substances.
Authorities said there was no structural damage to the site in Koltsovo, a small city in Siberia that has become a hub for biotechnology, molecular biology and virology.
Vector is one of just two places on Earth where the feared smallpox virus survives.
The other is a lab at the US Centers for Disease Control and Prevention in Atlanta.
The last known natural case of smallpox was in Somalia in 1977.
Since then the only known cases were caused by a lab accident in Birmingham in 1978, which killed one person and caused a limited outbreak.
The virus - which killed many millions of people over 3,000 years - was declared eradicated in 1980 following a global immunisation campaign led by the World Health Organisation.
Vector also researches and stores Ebola, HIV, Anthrax and hepatitis.
The lab announced the "gas cylinder explosion” and fire in a statement released on Monday.
The blaze was extinguished by local firefighters.
The head of the facility told Russian state media that "the incident does not pose any biological or any other threat to the population".
A scientist at the site died after sticking themselves with an Ebola-laced needle 15 years ago.
