The State Department has delivered a formal diplomatic request to Russia seeking more information about the death of 28-year-old Daria Shipilova, a worker at the Irkutsk Anti-Plague Research Institute in Siberia who died last week after developing pneumonia of unknown origin.

Secretary of State Marco Rubio said Tuesday that the United States is monitoring the case “hour by hour” and urged Moscow to provide more information. He said the details released by Russia so far remain limited and that greater transparency is necessary because of the potential international implications of a serious infectious disease.

The diplomatic request comes as U.S. officials examine whether Russia is meeting its obligations under the 2005 International Health Regulations, which require countries to notify the World Health Organization of events that could constitute a public-health emergency and provide relevant information.

Russia has rejected reports that plague was confirmed or that a laboratory accident caused the illness. Its consumer and health watchdog, Rospotrebnadzor, said Tuesday that none of Shipilova’s contacts had tested positive for plague. Testing instead identified two cases of COVID-19 and two cases of rhinovirus, while no other dangerous pathogens were detected.

Russian authorities also said there had been no accidents involving pathogenic microorganisms at the research facility. About 200 people were initially placed under medical observation, although most have since been released. The World Health Organization has said the international public-health risk currently appears to be very low.

President Donald Trump has offered U.S. assistance and said Monday that the United States would be willing to help Russia if necessary. Trump said Tuesday that he is scheduled to speak with Russian President Vladimir Putin and that the suspected plague case could come up during the call. He also said U.S. intelligence agencies are examining the situation.

Rubio declined to speculate about whether the incident involved an accidental release of plague or a deliberately modified pathogen, saying there is not enough information to draw such conclusions. The State Department has said it is coordinating with the Centers for Disease Control and Prevention and other agencies while urging Russia to share accurate information quickly.

There is currently no confirmed evidence that Shipilova died from plague or that a pathogen escaped from the Siberian facility. Washington is nevertheless pressing Moscow for additional details to determine whether there is any potential threat beyond Russia and whether international health-reporting requirements have been met.