An explosion occurred Monday night at a Russian biological research laboratory where samples of the smallpox virus are stored, reports said.

The incident took place at the Vector State Research Center of Virology and Biotechnology when a gas cylinder located on the fifth floor of the building exploded, a statement from the facility said. One laboratory worker was burned by the explosion and subsequent fire that spread over 300 square feet before it was extinguished. The worker was later taken to a hospital for medical treatment.

Reports said the disease research center which is located in Koltsovo, Siberia, also houses the deadly Ebola virus.

The statement noted that “no work with biological materials was going on there” at the time of the explosion and that no pathogens escaped the facility.

However, Dr. Joseph Kam of the Stanley Ho Centre for Emerging Infectious Diseases, told CNN that although facilities housing infectious diseases are under strict rules regarding storage, an explosion could potentially spread the viruses.

“Part of the wave of the force of the explosion would carry it away from the site when it was first stored,” he said. However, Kam added that because of the fragility of a virus, “more than 100 degrees or more will kill them.”

Reports said Vector is only one of two places in the world known to house samples of the live smallpox virus.

“The other sample of the deadly pathogen is stored at the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta,” according to the Moscow Times.

Vector was reportedly established in 1974 as a Cold War biological weapons research facility where vaccines and “defences against bacteriological and biological weapons,” were studied.

In 2004, female scientist Antonina Presnyakova died after she accidentally pricked her hand with a syringe containing the Ebola virus while working at the lab. Reports said Presnyakova was performing tests on guinea pigs when the tragic accident occurred.

“She was wearing the normal protective clothing, including rubber gloves, and got immediate medical attention, but the dose still killed her two weeks later. Four officials were disciplined at Vektor over the accident,” the BBC reported.

Her death raised concerns about the secrecy and safety of the laboratory but a 2016 inspection performed by the World Health Organization found that Vector met biosafety and biosecurity standards.

However, reports said that at the time inspectors “requested further work on some issues.”