Some of the most deadly viruses may have just been unleashed on the world, or maybe not — at this point we have no way of knowing.

Russian authorities — who don’t exactly have the best reputation when it comes to revealing accurate details — say a gas explosion has sparked a fire at a laboratory complex stockpiling viruses ranging from smallpox to Ebola.

However, the State Research Centre of Virology and Biotechnology denied that the fire had exposed the public to the pathogens stored inside, some of the deadliest on Earth, The Guardian reported.

The blast took place during repairs to a fifth-floor sanitary inspection room at the facility — known as Vector — in Koltsovo, in the Novosibirsk region of Siberia, the centre said on Monday.

The site housed secret biological weapons research during the Soviet era and is now one of Russia’s main disease research centres, the report said.

One worker suffered third-degree burns after the blast, which blew out the glass in the building. The fire reportedly spread through the building’s ventilation system and was later extinguished.

Russian authorities insisted that the room where the explosion occurred was not holding any bio-hazardous substances and that no structural damage was caused.

The mayor of Koltsovo also said that the laboratory did not contain any disease samples because of ongoing repair work — raising fears the Russians may not be revealing the whole truth.

The smallpox virus survives in two places on Earth: at Vector and at another high-security laboratory at the US Centers for Disease Control and Prevention in Atlanta. It is estimated that smallpox epidemics are wholly responsible for approximately 300-500 million deaths, through history.

Russian authorities last month were slow to release information about an explosion at a military testing site that caused a spike in radiation levels in Arkhangelsk region. The blast occurred when a liquid-fuelled rocket carrying nuclear materials exploded, killing at least five people.

Authorities initially denied the incident had occurred and reportedly did not tell local hospital staff that the victims had been exposed to deadly levels of radiation. Russia has not said what the military specialists were working on,  although experts have speculated it may be a nuclear-powered cruise missile.

Monday’s incident was not the first at the Vector lab. In 2004, a researcher died at the complex after accidentally pricking herself with a needle carrying the deadly Ebola virus.

Russian media then claimed it was the only death from the virus in Russia’s history. Outbreaks of anthrax and smallpox were caused by Soviet weapons development programmes in the 1970s and subsequently covered up by the government.

The Vector institute was threatened by a lack of funding in the 1990s, raising concerns that researchers could sell their expertise or actual biological samples to governments such as Iraq and North Korea.

The former head of Vector, Professor Ilya Drozdov, 63, also vanished in 2017 following accusations of fraud and a Soviet defector claimed that smallpox strains were moved there for bioweapons research.