U.S. Secretary of State Marco Rubio said his department is monitoring the death of a Russian lab worker who may have died from the pneumonic plague.

Oct. 5 (UPI) -- U.S. Secretary of State Marco Rubio said Monday his department is monitoring the death of a Russian lab worker who may have died from the pneumonic plague.

Rubio said he does not think there is cause for alarm. Last week, 28-year-old Darya Shipilova, an employee at an anti-plague research institute in Irkutsk, Russia, fell ill at work and died two days later.

Shipilova's death was being investigated as it was unknown how she fell ill. Russian media outlets have speculated that there was an accident at the laboratory that led to her becoming infected with the plague.

Rospotrebnadzor, Russia's official agency for health oversight, said Shipilova's death was related to a pneumonia infection "of unknown aetiology." It added that she was not found to have the presence of micro-organisms that were related to her work in the research lab.

Media outlets in Siberia say the facility was put into lockdown.

The Russian government denied that any accidents took place in the Irkutsk Anti-Plague Institute. It called the "sanitary and epidemiological situation," "stable."

Rubio acknowledged the reports out of Russia, saying that infection may have spread within the lab.

"There's very limited travel between the U.S. and Russia, obviously," Rubio said. "But like anything else, something that gets out, it just quickly spreads."