US Secretary of State Marco Rubio has declared that the US is closely monitoring the case of a researcher in Russia who died after allegedly being exposed to pneumonic plague.

The 28-year-old worked at the Irkutsk Anti-Murtarja Research Institute and passed away from an unknown pneumonia.

Russian authorities have said that so far there is no evidence that the disease was linked to the pathogens she worked with.

About 200 people who had contact with the researcher are being monitored, while an institution where she was treated is located in quarantine.

Rubio said the situation does not currently constitute reason for alarm, but requires constant attention and monitoring.

According to the World Health Organization, pneumonic plague is the most severe form of disease and can be transmitted from person to person through respiratory particles.

However, early treatment of antibiotics can be effective.

Experts say that if it comes to an isolated case, the risk of widespread expansion remains limited. /Telegraphy/