Secretary of State Marco Rubio said Monday the US is monitoring a suspected pneumonic plague outbreak in Russia, downplaying it as not 'cause for alarm.' 

Fears around an outbreak have spread after Russian scientist Darya Shipilova, 28, died days after reportedly dropping a vial of live plague bacterium.  

The incident reportedly occurred at the Irkutsk Anti-Plague Institute in Siberia last week. 

Shipilova told medical staff she had accidentally broken a test tube holding live bacteria, according to news outlet Lyudi Baikala. 

'There's very limited travel between the US and Russia, obviously,' Rubio told reporters on Monday before traveling to Iceland. 'But like anything else, something like that gets out and could quickly spread. So 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 to just keep an eye on it. And we're doing that.' 

Russian counterparts confirmed to US officials that the lab worker was infected. 

Russian authorities are insisting the anti-plague scientist died from 'unidentified pneumonia,' rather than the suspected bacterium famous for causing the 'Black Death.' 

Russia is believed to have implemented a lockdown on hospitals and medical centres in a 30-square-mile area following a feared plague outbreak.

Panic broke out when a female laboratory technician died and hundreds of people she came into contact with were placed in quarantine.

The State Department said that it is monitoring the outbreak with the help of  the Centers for Disease Control and Prevention (CDC) and other interagency partners.

A US official said in a statement: 'We encourage Russian authorities to share accurate information quickly and openly ... Plague is a known disease and is treatable with common antibiotics, especially when treatment starts early. Cases occur naturally in many parts of the world.'

Health and Human Services (HHS) referred the Daily Mail to the State Department's comment.  

The Daily Mail contacted the White House for comment. 

Plague is caused by the bacterium Yersinia pestis, and typically humans become infected through the bite of an infected host, such as a flea or animal. 

The disease is infamous for killing millions in Europe during the Middle Ages, when some estimates suggest it wiped out 30 to 60 percent of the continent's population, some 75 to 200 million people. 

While it is rarely fatal in the United States, it is deadly if left untreated and is known to still cause death in regions of Africa and Asia, per the CDC.  

Up to five hospitals in and near Irkutsk are in 'epidemic' mode, according to reports, with hundreds of staff and patients not allowed to leave.

Four hospitals have closed for a three-week quarantine, according to Lyudi Baikala.

Relatives of a woman giving birth at one of Irkutsk's maternity hospitals also reported to the outlet that it had imposed a quarantine.

Shipilova, the scientist who died, was admitted to a hospital on September 29 and placed on a ventilator but died two days later in Shelekhov, a satellite city of Irkutsk.

She came into contact with roughly 200 people, who are now being monitored over fears of an outbreak.

Regional authorities are implementing a 'comprehensive set of anti-epidemic measures', according to Maxim Modin, the mayor of the Shelekhovsky district in eastern Russia, near the Mongolian border.