The United States is monitoring reports of a suspected pneumonic plague death in Russia's Irkutsk region and has urged Moscow to share information "quickly and openly," as nearly 200 people who had contact with the victim remain under medical observation.
The Trump administration "is aware, monitoring the outbreak, and assessing options," an administration official told Axios.
Russian authorities have not confirmed that the woman, a laboratory worker at a plague research institute, died of plague.
Pneumonic plague can spread from person to person through respiratory droplets.
The case has prompted calls from some U.S. political figures for travel restrictions on Russia, though there is no confirmed evidence of wider transmission.
"We are aware of reports of a fatal case of suspected pneumonic plague in Irkutsk Oblast, Russia. We are monitoring the situation closely with the CDC and our other interagency partners," a State Department spokesperson told Radio Free Europe/Radio Liberty (RFE/RL), CNN and NewsNation.
"Many details have not been confirmed. We encourage Russian authorities to share accurate information quickly and openly," the spokesperson said, noting that plague is treatable with common antibiotics, especially when treatment starts early.
A Department of Health and Human Services spokesperson said the Centers for Disease Control and Prevention (CDC) is working with the State Department, adding that the safety and security of U.S. citizens is the department's "highest priority."
There are currently no direct commercial flights between the U.S. and Russia, and Washington has not announced additional travel restrictions.
The case centers on Darya Shipilova, a 28-year-old technician at the Irkutsk Anti-Plague Institute.
Independent Siberian outlet Lyudi Baikala reported that she broke a test tube containing pneumonic plague bacteria on Sept. 25. She was hospitalized on Sept. 29 with severe pneumonia and died overnight between Oct. 1 and 2.
The Moscow Times reported that she fell ill after returning from a work trip, while the Telegram channel Mash said she had traveled to Buryatia.
Russian authorities have not confirmed either account.
Russia's public health watchdog Rospotrebnadzor said the employee was diagnosed with "pneumonia of unknown etiology" and that expanded testing found no microorganisms linked to her work. It also said no accidents involving pathogens had been recorded at the institute.
"The sanitary and epidemiological situation in the Irkutsk region, the cities of Irkutsk and Shelekhov is stable," Rospotrebnadzor said.
Alexei Tsydenov, head of the neighboring Buryatia region, said the woman died "possibly from the plague," adding that "there are no plague outbreaks" in Buryatia.
Agence France-Presse (AFP) earlier reported that Tsydenov had said she "died of plague."
Irkutsk Gov. Igor Kobzev said those under observation showed no signs of illness and had tested negative.
"All identified close contacts are under medical supervision. At present, they show no signs of the disease, and their laboratory test results are negative," said Artem Fominikh, director of a nearby aluminum plant, adding that "there are many rumors and conflicting reports circulating."
Former Rospotrebnadzor head Gennady Onishchenko said the infection was "most likely not the plague, but even if confirmed, there will be no spread of this disease."
"To me, the most important question is, 'What was in that vial?'" Former White House coronavirus coordinator Dr. Deborah Birx told NewsNation.
"We do have good antibiotics that can combat it if treated early. But we have to know that it was not genetically modified in any way," she added.
Birx said the region is where Russia does "a significant amount of plague research," and that it would be "highly unusual" for someone in their 20s to die of pneumonia without an underlying condition.
"This would be very much a Russian issue, and I'm sure they're taking it quite seriously," she said.
Former CIA officer Dan Hoffman told Fox News that he expects the U.S. Embassy in Moscow to engage Russian counterparts "because we might have people in Russia who might be at risk," and that "Washington will be speaking to all of our allies and partners in the region."
Rep. Don Bacon, a Nebraska Republican, linked the case to Russian allegations of U.S.-backed biolabs in Ukraine.
"It was Russian disinformation ops at their best. But in reality it is Russia with the bio labs," he wrote on social media.
Former Rep. Adam Kinzinger wrote that "the whole world needs to ban ALL flights to and from Russia."
There is no public evidence that the Irkutsk laboratory was developing plague as a weapon or that the death was connected to a biological weapons program.
Plague can be cured with antibiotics when treated promptly, according to the CDC, as U.S. officials continue to monitor the suspected plague case.