Russia's public health watchdog on Thursday denied reports that a lab worker who died in Siberia had infected another person, as concerns continued over unconfirmed reports linking the case to pneumonic plague. Rospotrebnadzor said the woman, who worked at a plague research institute in Irkutsk in southeastern Siberia, died of pneumonia of unknown origin. It said it had informed the World Health Organisation that no case of plague had beened in the Irkutsk region.

In a brief statement, the agency said "extensive" testing found no evidence that the worker's illness was caused by any of the pathogens she handled. On Thursday, it also rejected reports that a second person had fallen ill after contact with the lab worker, calling them fake. The agency did not provide fresh details on the worker's illness and death, or explain the wide emergency measures taken in the region.

On Wednesday, Maria Van Kerkhove, WHO director of Epidemic and Pandemic Management, said the UN health agency did not know what caused the woman's death on October 2. She urged Russia to share more information about the case. According to WHO, it has sought further details to clarify the cause of the severe pneumonia, identify the pathogen that led to the public health measures, and examine media reports that a second employee had pneumonia of undetermined cause.

Van Kerkhove's remarks suggested the agency still had significant unanswered questions about the situation at the lab, which studies plague and was set up about a century ago to research the disease in Siberia, where it occurs naturally in wild animals. Rospotrebnadzor said on Wednesday that it had carried out the "whole range of anti-epidemic measures in connection with the professional activities of the person who fell ill".

It also said she had been "vaccinated and had immunity to dangerous infections".The agency added that an inquiry into compliance with biological safety rules at the institute found "no emergencies in dealing with pathogens" and confirmed that all safety measures had been followed. Russian authorities have released very little information about the case, while state television and other state-controlled national media have mostly avoided the subject. Some news outlets also withdrew their earlier reports.

Overall, Russian authorities have maintained that the lab worker died of pneumonia of unknown origin, that no plague case has been recorded in the region, and that reports of a second linked infection are false, even as WHO continues to seek more clarity.