U.S. Secretary of State Marco Rubio has urged Russia to release more information about the death of a laboratory worker at a plague research center in Siberia.
The incident sparked various speculations about the circumstances of the 28-year-old woman's death.
Russian officials said they were still investigating the cause of death, but that everyone who had contact with the deceased had tested negative for pneumonic plague.
"I think Russia clearly has an obligation to share more information with the world," Rubio told reporters. We are monitoring developments closely, hour by hour. "
Nearly 200 people were placed under observation over the deaths in Siberia's Irkutsk region, first reported on Friday.
According to Russian media reports, the laboratory worker was named Darya Shipilova, who was said to have fallen ill a few days ago.
Unconfirmed reports suggest she became ill after breaking a test tube containing the pathogen, or after a previous field trip where wild rodents roamed.
However, Russian officials downplayed the reports. The country's health regulator Rospotrebnadzor said the woman died of "pneumonia of unknown cause".
"No cases of this dangerous disease have been recorded in the city of Irkutsk," World Health Organization (WHO) spokesman Christian Lindmeier read out in a statement on Tuesday, in which Russia said.
But under global health rules, Russia is obliged to report the epidemic to the WHO.
The Russian statement added: "It has been reported that all close contacts of the patients have been identified and are being monitored for the disease, and to date, none have developed symptoms of the disease."
Ongoing speculation fueled debate between Russia and the United States on Tuesday, raising questions about whether the outbreak could spread beyond Russia's borders.
During his visit to Iceland, Rubio said the United States would take "measures" to help Russia if the situation in Russia was more serious than claimed.
In response, the Russian Federal Service for Supervision of Consumer Rights Protection and Welfare (Rospotrebnadzor) immediately expressed its willingness to provide assistance to the United States.
"The Russian Federation is ready to assist the United States in combating the epidemic of infectious diseases," the agency said in a statement.
Rubio's comments were echoed by US President Donald Trump, who said he was watching the case "very closely."
According to the Washington Post, the U.S. Centers for Disease Control and Prevention, the State Department, and the Strategic Preparedness and Response Administration are formulating U.S. response measures.
Asked about the deaths after Rubio's speech, Kremlin spokesman Dmitry Peskov cited the findings of Russia's Consumer Protection and Welfare Supervision Service.
Asked about the aid provided by the United States, he said "mutual assistance is a good thing."
On Tuesday, the Russian Federal Service for Supervision of Consumer Rights Protection and Welfare also issued a statement aimed at quelling speculation in a document circulating online that Russia had allegedly begun to take "special anti-epidemic measures."
"We would like to clarify that the relevant documents and information do not come from official channels and are untrue," the agency said on Telegram.
In fact, the plague bacterium Yersinia pestis killed half of Europe's population in the 14th century and was known as the Black Death. The bacteria have been found in rodents in the area, such as prairie dogs.
More than 100 years ago, in response to the plague outbreak in the 1920s, the Siberian and Far East Anti-epidemic Research Institute was established.
Dr Simon Clarke, associate professor of cellular microbiology at the University of Reading, told BBC Radio 4 that pneumonic plague (Yersinia pestis), caused by Yersinia pestis, is almost always fatal if left untreated, but can be treated with antibiotics.
