The United States Secretary of State, Marco Rubio, has said the US is closely monitoring the case of a Russian laboratory worker, Darya Shipilova, who died last week after a reported accident at a plague research institute in Siberia.

The 28-year-old lab worker died of a suspected pneumonic plague infection, after which Russian authorities placed a hospital in quarantine and began monitoring nearly 200 people.

Speaking on Monday before leaving for a trip to Europe, Rubio said the US was actively monitoring the potential risk of a containment failure.

Rubio said, “We’re watching and monitoring it closely. 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.”

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

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

Shipilova became ill at work and died two days later, on October 2, at a regional hospital in the nearby town of Shelekhov.

It remains unclear how she became unwell, although reports in Russian media suggested that Shipilova may have been infected after a mishap at the laboratory.

Senior Russian officials have added to public disquiet by sending conflicting messages. Alexey Tsydenov, the head of the Buryatia region, which neighbours Irkutsk, initially wrote that Shipilova had died of plague before editing his post to say she had “possibly” died of the disease.

Rospotrebnadzor, Russia’s health watchdog, said Shipilova died from “pneumonia of unknown aetiology.”

It insisted that tests found no micro-organisms linked to her professional work and described the “sanitary and epidemiological” situation in the area as stable.

Also, the mayor of Shelekhov, Maksim Modin, said the inpatient department of a local hospital where the researcher was treated had been closed for quarantine, while people who had been in contact with her were placed under medical observation, The New York Times reported.

Earlier, Irkutsk Governor Igor Kobzev said he had held an emergency meeting with Russia’s chief sanitary physician over a suspected case of a particularly dangerous infection.

Officials in neighbouring Buryatia initially referred to the woman as having died from plague before changing the wording to say she had “possibly” died from the disease.

Russian authorities have since sought to reassure the public, saying the sanitary and epidemiological situation in the Irkutsk region remained stable.

Kremlin spokesman Dmitry Peskov also urged the public and media to rely on official information rather than rumours and speculation.

Pneumonic plague is the most severe form of plague and can spread from person to person through respiratory particles.

The World Health Organisation says it can become fatal within 18 to 24 hours if untreated, but early treatment with antibiotics is highly effective.

Experts have nevertheless said the reported case does not currently indicate a wider outbreak.

Ian Jones, a virologist at the University of Reading, described the incident as appearing to be a rare laboratory accident and said it was unlikely to spread further.

The United States and European Union have said they are monitoring reports from the region.

Russia’s anti-plague institute conducts research into several infectious diseases, including plague, which is caused by the bacterium Yersinia pestis.

Human plague cases continue to occur in parts of the world but are generally treatable when diagnosed early.