US Secretary of State Marco Rubio said the US is closely monitoring the death of a Russian laboratory worker following a reported accident at a plague research institute in Siberia.

The 28-year-old worker, Darya Shipilova, became ill at work and died two days later, while reports of a possible laboratory infection remain unconfirmed by Russian authorities.

Several hospitals and the research institute have been placed under quarantine, with officials monitoring contacts and introducing other precautionary measures across the region.

Russian authorities have given conflicting accounts of Shipilova’s death, with officials mentioning possible plague before the health watchdog said she died from pneumonia of unknown cause and that tests found no work-related microorganisms, The Guardian has reported.

Experts say a wider outbreak appears unlikely because plague cases are rare, contacts are being monitored and antibiotics are available, although the incident has caused public anxiety and international concern.