A laboratory worker at a plague research institute in Siberia died on October 2 in unclear circumstances, prompting Russian authorities to quarantine about 200 people.

Russian media named her as Darya Shipilova, 28, a worker at the Irkutsk Anti-Plague Research Institute of Siberia and the Far East. She was admitted to hospital on September 29 with severe pneumonia and died days later.

Russia's health watchdog Rospotrebnadzor said she died of "pneumonia of unknown etiology" and that no work-related pathogens were found in her samples. The agency rejected reports that she had dropped a test tube containing live bacteria.

One regional governor initially said she "may" have died of plague, then edited the statement. Local officials said 60 per cent of those quarantined had since been released.

Unconfirmed theories circulated on social media, including a lab accident and illness after travel. There is no public evidence supporting any of them.

The World Health Organization said the public health risk to the general population appears low. The US said it was monitoring the situation. A criminal case into possible safety violations has been opened, according to Russian media reports.