The US secretary of state, Marco Rubio, has said the US is closely monitoring the case of a Russian laboratory worker who died last week after a reported accident at a plague research institute in Siberia.

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

Darya Shipilova, a 28-year-old employee at the anti-plague institute in the city of Irkutsk, became ill at work. She died two days later on the night of 2 October at a regional hospital in the nearby town of Shelekhov.

It remains unclear how she became unwell. Unconfirmed reports in Russian media have suggested Shipilova may have been infected after a mishap at the laboratory. Officials have not publicly confirmed that account.

Local media have reported that the institute has subsequently been locked down, with staff forced to sleep on the floor and on chairs. “They’ve been isolating everybody since Friday – more than 60 people,” one relative, Roman, told the news portal Siberia.Realities.

He said family members were allowed to drop off food at a checkpoint while tests were carried out inside the building. Similar quarantine restrictions have been placed on several other hospitals in and around the city.

Rubio, citing what ‌Russians have released about the death, said it seemed a worker in the lab became ​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.”

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.

Authorities in the nearby city of Baikalsk briefly urged residents to avoid travelling to Shelekhov and surrounding settlements after what they said was unofficial information about a possible plague infection. They later deleted the post from social media.

“There are no obvious changes to daily life on the streets, but people are worried and anxious. Locals are angry that they are not being told anything,” said Elena Trifonova, a journalist with People of Baikal, an exiled Russian media outlet specialising in Siberia.

Despite the lack of a confirmed diagnosis, precautionary measures appear to have been introduced across the region. The US-funded Radio Free Europe/Radio Liberty reported on Monday that several hospitals in Irkutsk had been placed under quarantine.

The outlet confirmed that staff from Shipilova’s workplace, the Irkutsk Anti-plague Research Institute of Siberia and the Far East, had been kept inside the institute since 2 October.

Patients at Irkutsk city clinical hospital No 1 and the Ivano-Matreninskaya children’s hospital said relatives were not being allowed to visit and people inside were not permitted to leave.

On social media, local residents described some of the precautions being introduced.

Ulyana Gavryunina, who went on Monday to one of Irkutsk’s main maternity hospitals for a routine check-up on Monday, said doctors refused to allow her mother in, citing a quarantine imposed over the suspected plague case. Gavryunina said staff member checked her for flu-like symptoms and asked her to fill out a screening form for infections, on which “plague” was listed. She posted a photograph of the form on Instagram.

Elsewhere, a mask requirement was introduced at the Irkutsk aluminium smelter in Shelekhov. The plant’s director, Artyom Fominykh, called it a preventive measure, saying: “Better safe than sorry.” Over the weekend several public events in Irkutsk were cancelled.

Shipolova’s death has prompted intervention from senior federal health officials. Anna Popova, the head of Rospotrebnadzor and the country’s chief sanitary doctor, travelled to Irkutsk on Friday.

She attended an emergency meeting of the regional sanitary and anti-epidemic commission over what officials called a suspected case of a “particularly dangerous infection”.

Igor Kobzev, the governor of the Irkutsk region, urged residents to remain calm after meeting Popova to coordinate the response. “All identified contacts have been placed under medical observation. As of today, none is showing signs of illness and laboratory tests have returned negative results,” he wrote on Telegram.

Kobzev made no reference to reports that a laboratory worker had died.

US politicians have raised concerns about the incident, which follows longstanding and unfounded allegations by the Kremlin that Ukraine has secretly carried out biological weapons research with the support of Washington.

A Republican representative, Don Bacon, a senior member of the House armed services committee, wrote on social media: “I remember hearing from anti-Ukraine voices about their so-called bio labs. It was Russian disinformation ops at their best. But in reality it is Russia with the bio labs.”

Plague remains endemic in parts of Siberia and the Russian far east, where the bacterium persists naturally among wild rodents and their fleas.

But human cases in Russia are rare. Before the suspected Irkutsk case, the country’s last recorded human infection was in 2016, when a 10-year-old boy in Siberia’s Altai Republic contracted plague after helping butcher a marmot.

“This case here seems like a rare lab accident and is unlikely to go much further,” Prof Ian Jones, a professor of virology at the University of Reading, in the UK, said. “Although it is true that pneumonic plague is directly transmissible, the contacts of the primary case are known and can be treated if they show any sign of infection.”

He added: “Plague carries a huge history and is still present in many parts of the world, including Russia, but the living conditions associated with previously spread plague are no longer generally true, antibiotics are available and its potential is therefore limited.”