The United States is closely monitoring the death of a Russian laboratory worker at a plague research centre in Siberia amid concerns that pneumonic plague could have spread, US Secretary of State Marco Rubio said.

President Donald Trump said Washington was looking “very strongly” at the case but had not discussed it with the Kremlin.

“I don’t think it’s cause for alarm, but it is cause for focus,” Rubio told reporters, adding that the US was monitoring the situation closely.

The woman, identified by Russian media as Daria and believed to be in her late twenties, worked at the Anti-Plague Institute of Siberia and the Far East in Irkutsk. She had recently travelled to Buryatia, a region bordering Mongolia where plague occurs naturally among wild rodents.

Irkutsk Governor Igor Kobzev said the woman’s cause of death was recorded as “pneumonia of undetermined etiology”. He said no microorganisms associated with her work had been found in her biological samples.

Unconfirmed reports suggested she may have become ill after breaking a test tube containing the plague pathogen or during her field trip to Buryatia.

Russian authorities said no other institute workers had sought medical attention since her death. Russia’s consumer and health watchdog, Rospotrebnadzor, also said a biological safety team had inspected the institute and found no emergency situation.

Russian media reported that nearly 200 people who had contact with the woman were being monitored as a precaution.

Kremlin spokesman Dmitry Peskov urged the public to ignore rumours and rely on official statements from Rospotrebnadzor.

The bacterium that causes plague, Yersinia pestis, remains present in parts of the world and can circulate among wild rodents. Pneumonic plague is particularly concerning because it can spread between people through respiratory droplets.

Simon Clarke, associate professor of cellular microbiology at the University of Reading, said pneumonic plague was nearly always fatal if untreated but could be treated with antibiotics.

“People think, ‘Oh the plague, that’s gone, that went hundreds of years ago,’ but it hasn’t,” he said.

Clarke said the disease remained a public health threat in some regions and that studying it was important.

Asked whether the pathogen could have been studied for use as a biological weapon, Clarke said it was possible, but added that a natural infection was more likely.

The Anti-Plague Institute of Siberia and the Far East was established more than 100 years ago following a plague outbreak in the region in the 1920s.