The United States has urged American citizens currently in Russia to leave the country immediately amid reports of a suspected case of pneumonic plague in the Siberian region of Irkutsk.
The warning was issued by the US Embassy in Russia as authorities continue to investigate the death of a worker linked to an anti-plague research institute.
The US government has stressed, however, that the reported case remains unconfirmed.
The Centers for Disease Control and Prevention said it was monitoring reports of a suspected pneumonic-plague case in Irkutsk that resulted in one death, while noting that the circumstances surrounding the case remain unconfirmed.
The US Embassy said it was also monitoring reports of quarantine measures and hospital closures in Irkutsk.
It urged US citizens already in Russia to depart immediately and advised Americans planning to travel to the country not to do so.
The embassy cited not only the health concerns but also the longstanding US security warning for Russia and the limited ability of American authorities to assist citizens, particularly outside Moscow.
The health concern centres on the death of a young laboratory worker in Irkutsk who was employed at a facility specialising in plague research.
Russian authorities have not confirmed that the woman died from pneumonic plague. Reports indicate that the recorded cause of death was pneumonia of unknown origin, while testing has reportedly not established a confirmed plague infection.
The distinction is critical: there is currently no verified evidence of a confirmed pneumonic-plague outbreak in Russia.
Pneumonic plague is a serious bacterial infection that can spread between people through respiratory droplets during close contact. The CDC says it can be treated effectively with antibiotics when identified and treated promptly.
The World Health Organization has assessed the broader international risk as very low based on the information currently available, while continuing to monitor developments.
US Secretary of State Marco Rubio has also called for greater transparency from Moscow over the circumstances surrounding the death and the information being made available to international health authorities.
President Donald Trump has said he intends to discuss the situation with Russian President Vladimir Putin.
For now, the US advisory reflects a precautionary response to uncertainty rather than confirmation that a widespread plague outbreak is underway.
American citizens in Russia have been advised to follow local health guidance, avoid restricted medical facilities and seek medical attention if they develop fever or respiratory symptoms. Those potentially exposed have also been advised to monitor their health.
The situation remains under investigation as US and international health authorities seek further information from Russian officials.