A lab worker died after reportedly falling ill at a Siberian anti-plague research institute. (The Irkutsk Anti-Plague Research Institute of Siberia and Far East via AP)

Washington: The United States is closely monitoring the case involving a laboratory worker who died at a plague research institute in Siberia, Secretary of State Marco Rubio said on Monday.

"We're watching and monitoring it closely," Rubio told reporters before leaving for a trip to Europe. "I don't think it's cause for alarm, but it is cause for focus and a cause just to keep an eye on it."

A laboratory worker at the Irkutsk Anti-Plague Research Institute of Siberia and the Far East fell ill and died in undisclosed circumstances, prompting authorities to quarantine dozens of ‌people, and the head of Russia's health watchdog to fly to the region.

There were conflicting reports about how and where the laboratory worker, whose age was given as 28, had fallen ill, but the authorities said she had died of pneumonia.

Rubio, citing what Russians have released about the death, said it seemed there was a worker in the lab who was infected and there was concern the disease may have spread internally.

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

Plague is endemic to Siberia, location of the first known prehistoric human outbreak, as the bacterium that causes it, Yersinia pestis, is present among the small wild mammals in its vast wilderness and their fleas. It is rare for Russians ⁠to fall ill with plague, however, though cases in other parts of the world - notably Africa - occur every year.

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